Congrats on the possibility of owning a snake!
I own mostly cornsnakes, and after dealing with others I believe these are the best species to get started out with. The likelihood of problems is pretty minimal, as far as feeding and temperment are concerned.
Corns are a temperate climate snake, and don't require elaborate conditions to be met for them to be healthy and happy. Slightly above normal household conditions work just fine for mine. Low 80's on the warm end and mid-70's on the cool end, work pretty good. Or even in a room that's a constant temp works well too. My snake room stays a constant 83, and no complaints heard from them yet. Those temps are in F, since us Southern folk haven't quite picked up on the Metric bandwagon yet.
Corns are also more even tempered than other snakes. The hatchlings will get nippy now and then, but it just feels like rough velcro. I've had cat scratches do more damage than corn bites. But once they get used to handling, I rarely have a problem. Most of my bites have been as a result of being careless and my own damn fault.
My kingsnake on the other hand, enjoys biting. They seem to be more motivated by food, so if its feeding time and they smell it, they tend to bite at anything in case it might be food.
I do own a Ball Python, and they're a very docile snake as well. But why people recommend them as beginner snakes is beyond me. They require more warmth and humidity than corns, which having dealt with corns seems to be more work to keep it just right. And they can be rather finicky about eating. The one I've got eats sporadically at best. He'll feed consistently, every week for a month. Then take 2 months off, uninterested in anything. Which from folks I've talked to, they say I'm lucky. He's the only one I have, so I really dunno.
With the size of tank you've got, you could even get an adult cornsnake which would be even better. I prefer to buy older snakes, generally anyway. They're more docile, more predictable, and generally don't have a single eating problem. Plus in case something isn't just right with your setup, they seem to be a lot more forgiving of husbandry mistakes than a hatchling.
I have never owned a milksnake, but have heard mixed reviews as far as temperment goes. Some tend to be nippy, some are dog-tame. I guess it just depends on the species of milksnake you get and the individual snake itself. Which is true for most snakes, regardless of species.
There are quite a few Canadian members of the forum, and they even have their own Forum section down a ways. Maybe one of them lives locally and could give you added pointers and places to obtain whichever snake you decide to get for your son. I wish my mom had done that for me, instead I had to buy my own snake and hide it for a month before she even knew about it.
I hope I helped a little bit into what you were asking about. It sounds like you've got the supplies already accounted for. You might pick up the Cornsnake Manual, and the others which are printed from the same publisher. They contain a lot of good information, and aren't too terribly expensive. I would get a list of which snakes you're interested in and then just Google for their care sheets online. =)
Best of luck to you!
