Here are some tips that I would use, although ever boy is different and sometimes what works for one is detrimental to another.
1. Intensity - yeah, definitely up the intensity. However, if you asked a child at this age what intensity means they may give some vague answer and I doubt many would truly know what this means let alone how to put it into action. I use the words 'Fast and Hard'. I try to get him to constantly think about the words Fast and Hard. When he runs onto the field: think "Fast and Hard", when he gets the ball: "Fast and Hard", running to a breakdown: "Fast and Hard"... "Fast and Hard, Fast and Hard!!!!" This is definitely intensity, but is more action and behavioral specific.
2. TACKLE TACKLE TACKLE - As you know, a high percentage of boys at this age either don't know how to tackle or haven't yet developed the confidence to do it correctly. When picking a rep side, I would imagine having boys who can tackle is going to be a priority. Most boys focus on their attacking play - I've always had a strong focus on tackling and defense. In fact I personally would be trying to set his mind state to be seeking-out tackles rather than waiting for them to come to him. A good way in doing this would be to have the attitude of trying to lead the tackle count at the end of the game - it's not important if he actually does or not, but having this state of mind will increase his intent to look for and make tackles. Tackle like a beast.. tackle, tackle tackle.
3. Beat Your Opposition Number - You must be able to beat your opposition player. In your son's case: line outs, ball carry, tackles etc. He should make it a priority to stop his opposition player from running at all costs. This works two-fold: 1) stops the opposition player from looking good and 2) you look a better player in doing it.
4. Don't be an individual - It still amazes me how boys play when going for a rep side - This can work to your advantage: boys on the opposition team will run the ball a lot without passing - this can help you in defense as the ball carrier becomes more predictable e.g. don't fall for dummies and focus on the ball carrier because there is a high chance he is going to run it himself. And, most definitely play as a team player - run it when it needs to be run and pass it when it needs to be passed. Also, focus on ball retention (holding onto the ball tighter) as players will be more intent on ripping the ball from you.
Robbie Deans when talking about the recent State Schoolboy Championship, said: "... they'll be looking at players that make their team better!" This is really important for young players to know when playing. It's not about making yourself look better but how you make your team better.
This is a lot of stuff and if all this is going to make your son stressed, unhappy or feeling overwhelmed, I would disregard it and maybe just say: "have fun and play hard" which may very well do a better job. I tend to focus on one or two things a game, so a thought could be to pick a couple of things to focus on.
I wouldn't worry about selectors - good or bad. If he plays to the best of his ability (or close enough), has fun and still doesn't get picked; at least he can be extremely proud of himself and walk away with his head held high. The rep. games will come and go, it's more important what he takes from it and have fun doing it.
oh.. one last thing: I think it's really important to play your position - I've seen players (in an attempt to please selectors) not do the job they are suppose to do. But to play your position should be highly regarded. Goes without saying I guess.
Good Luck