ETA: this post is very opinionated, but you know your DS and I don't. Please take what works and leave the rest!
1. He will be in good company. They are going to be at least 6 kids who will be about the same level as he is. The teacher is planning to get these kids to finish the regular problems (maybe about 15-20 problems). Since these will be "easy" for these kids, they should be able to finish them quickly, and then they will be given the harder problems, most likely, problem solving.
Sounds good, provided your DS will work fast enough at the "easy" problems to have plenty of time for the hard ones. Maybe have him time himself and use it consciously for speed/accuracy practice?
The math specialist told us that based on what she knows about the kids, she highly recommends the pre-algebra since these kids will be at higher level than the algebra 1 class being offered at a later time.
This is very plausible; there really isn't much stuff in Algebra I. All the interesting mathematical development potential of either class is in the problem solving. Your maths specialist sounds like a gem, actually :-)
3. I have re-read the "Calculus Trap" article in AoPS website, and the discussion of that article in this forum. If we apply what Richard Rusczyk wrote to DS' situation, does it mean that he would really be better off being in pre-algebra with lots of problem solving skills added to the class than taking algebra 1 now? How relevant do you think the article is to HG / PG kids?
In a word: yes. And it's definitely relevant in its main point which is: it really doesn't matter when you learn a particular technique; what matters is how good you are at solving problems. (I think RR understands HG+ kids much better than ND ones, actually - his main focus is on those doing maths competition at a high level, and I imagine there's a correlation there!)
If in prealgebra your DS will not learn any techniques new to him - he'll be there just for problem solving and for speed/accuracy practice - you may find at some point that he's desperate for new stuff, and then maybe he can find some in the AOPS book you have at home. His not having done much maths over the summer speaks against that, though, I think. I see it as being rather like "let the kid trust their body" when it comes to eating. Sometimes they're hungry for new maths stuff, sometimes they're not. Right now it sounds as though although he wanted to be in Alg I, it wasn't really because he's desperate to learn new maths, or he'd have been doing that over the summer. It may be just the right time to stop taking in new stuff and focus on gaining maturity with what he's got.
