Tink20 -- I think he'll be fine. First of all, he can choose to wear a shorty wetsuit, which provides a little bouyancy (and warmth, the reason I opted for one). There are plenty of flotation vests available, if that'd make him more comfortable in the Grand Reef area (the water's a little deeper there and because of the fish and rays, can be a little daunting to a kid). I have an 10 year old who is a so so swimmer and gets anxious, and it took him a little time to warm up to the Grand Reef, but he took his time and eventually loved it. All three of my kids (then 10, 8 and 6) loved DC and said it was their favorite day out of the whole WDW vacation. (The 6 year old did not swim the grand reef though -- the rays scared him, but he was very happy making sand villages at the water's edge with some other kids his age.)
The lazy river is really relaxing, nothing to make a weak swimmer nervous, and has plenty of places you can "pull to the side" and hang on some rocks by a little waterfall, etc. And lots of lots of noodles everywhere, with almost everyone taking advantage of them (so no need to feel embarressed, which I mentioned since kids care so much about blending in).