I agree with a lot of what has already been said. I just wanted to address a couple of things that you asked.
Is it wrong of me to find this suffocating and unpleasant?
Definitely not wrong. It IS suffocating and unpleasant. No reason to beat around the bush.
Do autistic people behave like this or is he making excuses?
I've found this to be a red herring so often. Being autistic is not an excuse for bad behavior. Rather, it's there to let you know how to go about making yourself understood to someone who may think a little differently.
It is never, never, never, never, never OK to let a behavior that genuinely makes you feel uncomfortable fly just because you feel as though you're supposed to put up with it for any
reason. "But, he's has Asperger's" or "But, he's mostly a good guy" do not
negate the fact that he is continuing to do things that make YOUR playtime unpleasant.
How can I get him to stop it without being rude and making him feel bad?
These two things do not go hand-in-hand. They never will. And it's best to stop equating them.
There are a myriad of ways to politely enforce your boundaries, and to get this man to stop annoying you.
These perfectly acceptable and polite tactics may end up giving him a case of the sadfeels.
Let's face it. Hearing from someone that you like that "Hey, you're smothering me" no matter how politely worded sucks a bit. You know what a normal response to that would be? "Well, that sucks. I'm really sorry that I didn't realize it, and that I've been making you so uncomfortable. I'll back off."
(Granted, an even more normal response is not sending sad faces to someone every.single.time. that they're busy elsewhere.)
His temporary sad feelings about being told of his own annoying behavior are HIS problem. That is not your problem.
It is not, and will never be your problem to police another person's feelings. You cannot stop him from feeling bad about this. You are allowing the threat of his potential bad feelings to stop you from enforcing your boundaries.
I don't mind chatting with him but not every single day. I also don't appreciate that I can't talk to other people without him joining in or making sad faces at me. Some advice on how to get him to back off would be really nice, please.
Again, some really great advice has been given, so I won't add to that specifically. Mostly, I just want to say that while it's nice you want to consider his feelings, it's pretty obvious he hasn't considered yours. So firm up those boundaries. You don't have to allow your downtime to feel invaded simply because you don't want to "be mean". Rest assured. You're not being mean.
You're going to be polite, but firm.