Ahhh ... the golden question. Just out of curiousity, is he very dedicated to his saxophone playing? If he does ... I would only suggest this ...
1) If you are so darn sure he is going to stick with it for a long, long time, might as well get something that will last him! In other words, an expensive sax! I don't normally recommend this, but get the most expensive one you can afford for him, and it will create psychological guilt on him if he doesn't play it

- I would suggest modern horns like P.Mariat horns as they are fab modern builds. Or a Yamaha-62. Now THAT is a horn that will last him for life. I wouldn't suggest you getting him a vintage horn for now as, well, it's vintage for a reason. I am not saying that they are bad, i am saying that they comes with their own quirk and these modern horns are designed to maximise one's learning potential. Still, i started on a vintage and it worked for me so ...
2) Get someone whom is experienced to go try the horn with you. Better yet, get your son to go choose one himself! That way, he will be able to decided what is best for himself, and what's his style is.
3) If you're keen on budget, Yamaha YAS-23/YTS-23 are the way to go. Solid beginner saxophones, easily last him to even expert stages and can be used as a back up anytime.
4)And lastly, please keep in mind that the horn chooses the player, not the player chooses the horn.
Hope this helps.