I think you're on the right track with your original ideas. The 540A integrated would probably be ok too especially as your son will likely be using his system in small rooms for years to come. I'm wondering if he might want a tuner. A lot of young people listen to radio frequently. If so a nice receiver along the lines of the Denon DRA-395 might be a good choice (there is a review in the product reviews you could check). I'd also consider a dvd player for a digital source. Most kids like video and the ability to play concert dvd's might be a welcome feature. A system like a Cambridge/Denon amp/receiver, Epos speakers and say a Pioneer multi-format dvd might be just the ticket and leave some bucks left over to get some starter source material too! Anyway I think your son is a pretty lucky young man and I hope you guys enjoy choosing and playing his new system.
Gary
Gary