If I had a 5 handicap, I wouldn't change a thing.

I've used NXT and DT Solo when I sliced a lot more often to get some less spin to keep more balls in play.  Nice distance then but I don't see enough distance differences now on solid, straight hits with 2, 3, or 4 piece balls to try to get more distance.  I'm still working on my best ball since I improved my ball striking (...stopped a reverse tilt).  DT Solo can balloon some.  Depending on your son's launch you can look at ProV1 or NXT Tour/Tour-S for distance but you might be messing with his short game.

The Pro V1x is a great ball. From tee to green. Perhaps you could get him a sleeve of Pro V1's, the new Velocity, DT Solo, and the NXT's. Try them all out.

If it's possible, see if you can get a hold of your local Titleist rep and have him/her, put your son through a Titleist ball fitting.

I would go with the DT Solo. It has much less spin all around which should give him a few additional yards. He won't be able to stop a DT like  ProV though.

What is his driver loft? If it's more than 10 I would look into that as well. I would look into getting him into an 8.5 or 9 degree. I was started with an 8 degree driver when I was 10 and it helped me be very accurate with my swing (less loft means less room for error).

My personal thought is this, if you can afford the ProV1x...stay with it. It will out perform any ball you find.

I have a 9 year old son who is 4' 9" and has a swing speed of 75mph with his driver (Cut down 910 D2 with Bassara L shaft).  We tried him with a all the Titleist balls.  The ones he liked best for feel was the Titleist Velocity.  He also liked the NXT Tour S as well but that did not quite have the same distance off the tee.  Both balls have good green-side feel and are quite a bit better IMO than tthe DT Solo.  Also the Velocity is very reasnobly priced.

The NXT Tour is a good alternative due to the fact that the ball is relatively cheap and gives good distance without creating much driver spin(which will give him more distance)