Whoa! Props to you and your little guy!!!
Our girls are now 8 and 10. When they were your kids' age, I seriously cut back on my kayaking, bought a raft, and started running more I-II stuff....and spent a TON of time with them just swimming and playing in the water. They couldn't swim without life jackets until about age 4 and 6, but are fish now. I didn't want to push them into kayaks, so I waited until they asked--and last year, they just did.
You can find the Stearns stuff at a regular sporting goods store or even Target. If he's swimming and diving, consider the non-collar PFD. Regular kids' Type III with the crotch strap.
I have to say it again--your guy's swimming talent is IMPRESSIVE!!
Also use "peer pressure" to your advantage. Find other boating parents and find him a playmate who also likes to swim. You'll find they swim/dive/play even more aggressively if they're doing it to keep up with or outdo another kid--or simply get the confidence to try something new knowing another kiddo can do it.