How to show my male friend I don't fancy him. He avoid me because he thinks I do!

I met this guy through friends and I always though he was gay even though he had a girlfriend as people told me he was. I was really friendly with him and complimentary. If I had known at the time he was straight I would have avoided doing so as I would have known he might take it the wrong way. After knowing him for 6 months he came up to me and said he wanted me to know he was not gay as he knows people sometimes say he is. I believe him and I also presume he is reasonably happy with his girlfriend although he sometimes puts down their relationship.

The issue is he largely ignores me when we are out in a group together. If we happen to be walking together or are left alone he will always walk away. He is never nasty but just avoids most interaction with me. He's a decent guy and I can't have done anything to upset him so I honestly think he thinks I fancy him and doesn't want to give me the wrong impression lol. He makes a massive deal of pointing out hot girls when I'm in the group when my friends say he doesn't do that when I'm not there. I've tried to let him know that I'm not interested by no longer complimenting him and just not being as friendly but it's so awkward with the two of us now and I really need him to know I don't fancy him and won't be asking him out or trying to snog him if he's alone with me!! haha. How do I best go about this?

He's always been like this, didn't want to be alone with me as unintentionally I think I've come across as interested in him.

Of course he might just really not like me and something about me annoys him. But if I can eliminate one possible scenario and know he just doesn't like me that would be good.

I'm a girl. See I'm certain if I ask him he will only say hes not avoiding me. And then it will be even more awkward. But we can't just have it the way it is. It's uncomfortable and affecting how I feel about meeting up in our group.

If you don't like him, why do you care what he thinks?

It's like The Weeknd says there's a room full of niggaaaaaaaaaas, what you bothering me foooooor.

It's because we have mutual friends and its awkward. I want him to be comfortable around me or at least civil and not run away whenever we have to be alone. It's bizarre/rude. If it's something as simple as him thinking I fancy him it's rectifiable if I can find a way to show him I don't see him that way. I just don't know how to do it.

Are you Polish?

nope. british

Ah that's difficult. It just that I know light-skin Polish girls go to rooftop parties with guys, and if you uploaded the pics to insta he would get the message.

this would work and despite how stereotypical this sounds OP it is true I used to sell vodka for these events

oh really, how did you get into that pal? (if you don't mind me asking)