I don't think you can change sizes. He won't need any clothes other than socks and underwear. They are given enough shorts and tee shirts for each day plus a sweat suit. They do need sheets, pillow and towel.
Agreed on all counts. The t-shirts and sweatshirt are pretty much the same size as any other unisex t-shirt you might get from an trip/event/team order etc. The dri-fit shirt is a bit looser, but not a whole size worth. I actually wore mine to practice today. The shorts are also about the same way, loose and roomy like most other gym shorts, and they do have a drawstring if they're too loose. The sweatpants as far as I can tell were quite roomy on everyone, which is fine until they're water-and-sand-logged from the adventure in the Severn.
When they were distributed when we arrived I seem to recall that they weren't overly fond of people changing sizes, so do be a bit careful not to select the wrong size.
I brought many clothes with me that I never used, but did use all my pairs of shoes, (2 pairs of sneakers plus my Vibram 5-fingers). I did dispose of the set of old sneakers that accompanied me into the Severn.
Definitely bring plenty of underwear and socks, as well as the mentioned sheets, pillow, and towel. If your son likes to be warmer when he sleeps, throwing in a blanket would be a good plan too (something I forgot and regretted in Bacroft's AC and the relatively mild weather of the week I attended NASS).
I've kept all the clothes I got last year (I chose to rescue the still-stained sweats) and wear them to practice quite often, the shorts in particular.
I would normally lend a hand with the laundry but now that I know this I just have to find an excuse. A pricless moment of fun all for me!