He seems to struggle with how far he has to stand back and getting him to run in diagonals when a winger or centre breaks rather than run the width of the pitch and chase from behind ( i can't describe what i mean very well, hope you can picture it).

Like I said then, just tell him to roughly try to stay in line with the ball at all times and to stand deep enough so that opposition players can't get around the outside of him. Professional teams operate with different defensive systems and obviously at a much more complicated level than what your son is playing at, so trying to copy exactly what they do in defence might confuse him a bit. The most important thing is for him to learn to anticipate where the ball is going to go and so where he needs to be in relation to this. All professional fullbacks will do this naturally without having to think about it, and pretty much everything else they do comes as a result of that.

cheers pal

what head hunter says. Plus watching a full back wont help as when you see a full back on screen they have already moved into position.

I would also kick ballsfor him to practice his catching.

Question: If a full back isn't the last line of defence, who is?

What he said. Roughly in line with the ball without overcommitting.

It's unlikely an attacking team will kick early in the set so he can stay close enough to the line (I would guesstimate 10-20 metres) to intercept any breaks early. On tackle 4 he needs to drop deeper (30-40 metres) and remember to instruct his wingers to drop on 5, or 4 if the coach prefers. A big point for a full-back (indeed any defender) is that the touchline is his friend; he never needs to be too tight to the touchline when positioning himself in general play.

Learning to run an 'intercept' line is tricky at a young age. I would try and get him to understand that he's not trying to catch the attacker, but to cut him off from the tryline, and/or shepherd him to the touchline where has has nowhere to go.
As a simple exercise, roll a tennis ball on the floor and ask him to roll a ball from another angle to intercept its line before it gets there. He'll have to roll his ball ahead of the ball in motion. That should demonstrate the angles he's looking for.

And practice, practice, practice catching and tackling. A good tip for open field one-on-one tackling is again, not to overcommit. Don't sprint at the attacker and hurl yourself at him, any decent runner will just step. Show him you're a calm presence closing him and his angles down and he'll have to either make a decision or take the tackle.

If you also want him to look at joining the attacking line then yes, watch players like Slater and Wellens. They will form a second or third phase of attack, hitting the attacking line as the other players complete their moves or runs. Full-backs often position themselves behind the play-the ball so the defence doesn't see them and therefore doesn't count them when matching up defensive numbers. The full-back will then sprint round the back of the line as the ball moves through hands and inject into the line as an extra attacker, hopefully outnumbering the defenders.
Running this sort of stuff as a set move is fairly advanced so I'm not sure if his coach will be doing that with 12 year-olds, but there's no reason he can't take it upon himself to practice this as a support player.

The best role model to study Full Back play will probably be the last 10 years of Paul Wellens. His all round game in defence and attack is still at a high standard, to make up for his loss in speed he reads the game very well and is often seconds quicker in thought in commitment to the tackle or offence.
There is always debate on who was the better full back between Radlinski and Wellens, for me Radlinski was quicker in attack and Wellens better defensively.

Never heard that one before, "a full back isn't the last line of defence'.

I think the point people are trying to make about attack vs defence is that defence is a minimum requirement, but the fullback in the NRL is also one of your key attacking players. SL (but also British RL generally over the last 30 years) simply hasn't produced good attacking fullbacks. A great 'defensive' fullback wouldn't get a gig in the NRL. They need to be able to add to the attack to have any kind of career.

The game is changing, and we should all want to see more Tomkins-style fullbacks in SL, and fewer Keith Mumby's (just using him to avoid mentioning a certain formerly curly haired Sts player).

First line of attack AND first line of attack.
The first carry out after the kick is hugely important and a fullback with pace and balance can swing a game in seconds

Hi - my lad is FB (what age is yours?) Mine's U13's.

He practices 3 days a week (outside normal training) on his ball handling, catching high bombs, grubbers etc. He's also a kicker, so he will practice and practice this - especially drop outs, and variation at kick offs.

At training he focuses on 1 on 1 tackling, retrieving and running the ball out, creating space, making plays, attacking drills, organising the defence and positional play. He sometimes plays Loose (which personally I think is better for the current team), but I guess it's important to be flexible. He's also played wing, centre, srum h and stand o, but his preferred position is FB (probably cos of his early idol - Radlinski).

What do his coaches advise? Do they do specialist back drills/backs play etc?

Who's your local team? Do they do summer camps? Wigan did a specialist backs camp a few years ago with Barrett and Tommy L he found that really useful at the time, but sees himself as a bit old for that now.

He watches every match on TV he can - SL and NRL, and old games on youtube and is basically obsessed. I though he'd have grown out of it by now but nope.

Footage may not be easy to come by, but Steve Prescott would be an excellent to to study, especially the way he could pull off a cover tackle and take a high ball. He was also an excellent attacking footballer.