5/5 A+ 18yo PF, that is quite possibly the best scoutee to find. He could be just a glorified C with JS, but he could also be some insanely well-rounded player with HOF potential who could be a killer LI SF/outside PF. I would love to be in your position right now.
To answer your question, I'd wait and see his skills. If he has really well-balanced skills (5-7OD, 5-7JR, 5-7PA are things I would be looking for) then I would definitely keep him, not sell him. Such players can be trained into monster well-balanced SFs which are extremely hard to find on the market, and if you do find them they can cost literally millions. If he doesn't have well balanced skills (say something like 7 JS but 1-3 OD, 1-3 PA, 1-3 JR) I would sell him. Someone will pay you a lot for him, and the money will come in very handy. And serviceable pure inside players can be picked up relatively easily and inexpensively, unlike the well-balanced players who can play both inside and outside.
If he does have good skills and you decide to keep him, I would get at least a level 4 trainer, or a level 5 if you can get one. Beyond that is probably too much to reasonably afford. Technically there has been analysis that level 4 is the most cost-effective, but level 5 will give you 1-2 more pops per season and if you're trying to train a well-rounded player, it will take many seasons to train his skills up and you'll need all the advantages you can get. Those 1-2 extra pops in his 18yo season might save you having to train him for 6+ weeks when he's 27 and you're trying to finish off molding him into the perfect player.
Recap:
1. Wait and see skills before deciding to keep or sell.
1a. Inside skills are almost certainly going to be high because he has an A+ grade. So outside skills should determine whether or not you keep him.
2a. If outside skills are good, keep him, as finding a replacement can be difficult and expensive.
2b. If outside skills are bad, sell him, because the money will be more useful and replacements are cheaper.
3. If outside skills are good and you keep him, at least get a level 4 trainer, preferably a level 5 but only if you can find a cheap one.
4. Give this post a +1 ball. (Shameless, I know.

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Hope this helps!