Wood or GRP?

Sounds like your lad is a very accomplished sailor. I remember one Giles Scott coming training with us when he was 15 - he was 6'6" even then.
I'd say that your point about emotional maturity is very valid, however I think too much emphasis is placed on getting into squads, rather than why - I would say he sounds talented and he has a lot of varied experience - not many squaddies are as rounded. Make sure he is enjoying himself where possible, and that to work on every area of his sailing and te squad stuff may take care of itself. Quite a lot of senior squad sailors were youth squad rejects.
If he wants to realise his full potential he is going to have to suck up disappointment/rejection, and learn how to stay interested along the way.
My advice is to get something fun or engaging as light relief but he will be over any disappointment quickly - losing a season of laser competition may be a shame at this stage in his development.
Be careful about allowing him more than a sniff of beer, kitesurfing and fanny though, that could be career ending!
Something with a conventional chute (dying art in dinghies) that will be able to take a little weight and have some performance would be good.

You could do worse than get him out in that B14 you're trying to sell....
If he's going to step aside from Laser type racing for a bit, it might pay to do something fairly different, rather than just another una rig singlehander with a smaller circuit following.

I think it pays to consider where you are going to sail, including what opens etc, who you are going to be sailing against and what you can learn from them.  It's not entirely about boats, it's about opportunities to race. Where are the people you want to race against, and what do they sail?

I agree with other posts about the Solo but you are not going to get competitive boats at that price: they hold their value too well.  So that way lies frustration.

Well I read it all, kept biting my tongue and hands off the keyboard as the usual suspects flogged the dead solo and tried to polute a young mind with wood bothering, but I'm going to seriously suggest something else if you want to clear his head ready to return to the pursuit of Olympic glory later down the path, I'm going to suggest you let him have a bash at Racing Sailboards, he sounds like he's the right build, they aint expensive, they are fun and he'll learn wind awareness that becomes totally instinctive beyond anything he'll pick up in a dinghy, he'll also have to get a bit fitter.
Then when he returns to what is it he'll go into a Laser? It'll be with a new awareness of what he's dealing with, he already has all that stuff I don't, the rope and wiggle stick control, if he then gets the senses you pick up from the sort of 'hands on' no compass, no telltale use your senses only sailing you have to learn on a sailboard I'm sure he'll end up a better sailor for it..

Just a left field thought, other than that I think what you're doing with him is a good idea, all that squad system does is mess with their heads and creates a pecking order that becomes difficult to break.

Dart 15 is an absolute blast! I had one and even at only just touching 11 stone I could just about handle it in full 'Sport' mode single-handed (Jib, Main, Trapeze). I didn't take it class racing, but there looks like a good set of well supported events. They can be picked up fairly cheaply and parts are easy to get hold of - Steve Sawford at SpeedySports is very helpful (The new 'DX' sails look awesome!).