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.