What boat? If you aren't sure, take a picture of any logos of it. If it is long, wide, and has a rather pointy shape, read the next 3 paragraphs. If not, skip those.
Most recreational kayaks (the ones you would buy in a store that sells golf clubs, groceries, etc) are fairly wide as is, and the seat is as wide as you could go. When I was selling my first kayak, I had someone who wasn't that big that physically was too wide for the rim, but it was 19" wide.
I know that you want it to be a surprise, but it's really important for it to fit properly, and would be worth having him look at a few. You could probably drag him along to a store, and see if he shows interest in kayaking anyways. Leave him there while you shop, and when you are done see what he likes.
Just a warning though: some boats have hatches, but I know the Pelican brand kayaks sometimes have a hatch that is literally big enough to carry your lunch, and that's it. I would either get a really cheap kayak to start with (Victory Blast comes to mind), or go up to a Necky Manitou Sport (12', decent speed, big hatches). I had a rec boat a few years ago (sold it to make room for another whitewater kayak) and remember looking at the Wilderness Systems Pamlico 100 since it was only $425, but the hatch doesn't have a foam wall to keep it dry, so it wouldn't be too functional (although I recall Austin Kayak selling the foam "bulkhead" that you could glue in to keep water out).
If it is a whitewater boat, there's not much you can do, since you can't make the rim wider unless you are very good at welding, and even then it would void the warranty. The first boat I would look at in this case would be one of the largest Jackson Kayaks (Mon-Star, Superfun, Mega Rocker, and Super Hero come to mind). Their boats are basically stretched out for the bigger models, which means the rims also get bigger. The Superfun is at least 2" wider than most other boats ("standard" would be from 33x18 to 34.5x19.5, but the Jacksons are closer to 37x22 or so).
Depending on where you are will probably dictate which boat you get for him. If you are in the mountains, with plenty of whitewater rivers, a creekboat is a good spot to start (something with lots of "rocker", where it looks like a banana from the side, and no "edges" that may flip him upside down). If you are in a really flat area, then look more towards a recreational kayak.
If you plan on getting a recreational kayak, I would be more inclined to suggest a sit-on-top kayak like a Liquid Logic Coupe, Fluid Buddy, Wilderness Systems Tarpon 120, etc. They are more simple (less things to break), won't make him feel claustrophobic or "trapped" inside, and if he flips he can get back on it easily. The only disadvantages of them are that you can't roll them easily (if he wants to learn how to roll it back upright), and you are exposed to the elements (sun, rain, bugs, etc).
If you have any more questions, just ask!
Ian