Know Something About Snakes?

http://en.wikipedia.org/wiki/Snake may be of use.

When it starts to get cold out, he'd definitely start feeling sluggish and very tired. He'd actually find it difficult to move at all as his body begins to shut itself down to preserve his life processes during the 'cold time'.

So he no longer generates his own body heat? He can wear as much in the way of clothes as he likes, but that will only insulate him so far. He'll still be losing heat since even thermal clothes bleed off heat, albeit at a slower rate than jeans and a t-shirt. If he generates no body heat at all, then clothes can, at-best, retain some heat from earlier in the day. Eventually, they'll be just as cold as the rest of the surrounding environment.

Moving around a lot doesn't generate much in the way of kinetic energy in snakes. Our internal heat comes from our metabolic processes. As those processes increase (like during exercise) so does our internal heat. Reptiles don't have that kind of ability at all. However, it would be very human of him to think that the things he does to stay warm as an endothermic human would work as an exothermic reptile.

Does he still sweat? Sweating would be absolute death unless he was in a warm environment. Even then, I'd think it'd be uncomfortable.

A hot shower would probably get him back to feeling 'human'. Heh.

I can only speak from experience with a corn snake. They move very slowly when the temperature is too low. They are most active when they're warm and when they're hungry. Holding one when it's too cool for them, you can feel that their flesh is cold, and it's almost like their muscles are stiff, they're barely moving. If you continue to handle them, you can feel them start to warm up, and they'll move around a bit more, but it's not instantaneous. (This might be the case with your character if he tries to warm himself with someone else's body heat.)

A heat lamp is the best thing to warm them up. The problem with a snake trying to warm itself with a heated object (like a hot rock) is that the heat is too concentrated. It'll burn part of its body trying to get warm while the rest of its body remains cold. Ideally, you're trying to heat the air around the snake. I'm not sure what temperature diamondbacks need, but since they live in North America, I can't imagine it's that much higher than what some humans like.

As for research, you could try a pet store. Look for books about snakes, or better yet, talk directly with someone there who knows about them. (Not all pet stores carry reptiles, so you might have to look around a bit.) Some stores may let you hold them, most should let you watch them eat, if it's feeding time. You probably won't find a diamondback, but you should still get some helpful information about snakes in general.