Disclaimer: I am also autistic (high functioning).
It sounds like you are a bit higher functioning than the person you describe, which may be part of the problem. As you know, we have a spectrum disorder which is diagnosed by meeting criteria from several categories and to varying degrees.
As you've learned, just having the autism in common doesn't mean you have much else in common, and sometimes even the flavors of autism don't jibe well.
Given that, the problem sounds like he is mistaking social cues. If you want to maintain a friendship with this person, be very direct and say
"Please don't comment on my appearance, it's not appropriate and it makes me uncomfortable."
Be direct, be firm and leave no room for ambiguity. He may have it in his head that he's complimenting you and may not be able to judge the social context. I know that I run into problems myself with the connotations of words and can say inappropriate things myself, so I can see how someone could get into this innocently.
Your consideration for his feelings is admirable, BTW.
If you just don't want to be friends with this person any longer try something like:
Dave, I'm sorry but outside of the autism, I just don't have anything in common with you. I just don't see any connection, even at a friend level.
See how that works. If he still doesn't get it, then you may have to make it a bit stronger.
Dave, you're a good person, and I don't have anything against you, but I do not want to be friends with you.
Then let it rest.
If he still persists, then you may need to take a stronger stand, and even block him from social media if need be, but use the minimum force you can.
- SET CLEAR BOUNDARIES (he may not understand).
- State clearly your intentions.
- Take progressively strong measures.
- Let him know that he's making you feel uncomfortable, and why.