How can I tell if he is into me? How can I give him subtle signs that won't scare him away?

So I have this friend of a friend that I have made recently in the past year, and over the time that I have come to know him better, I have found out that I really like him. Ideally, I would want our relationship to strengthen as friends first and foremost, but I would also like to explore something that goes beyond friendship with him. However, I am not sure how he might feel about it. He seems to be pretty comfortable around me when I sit close to him, and whenever I talk to him in a flirtier tone, he doesn't seem to shut the conversation down in any way, so there may be something there. Also, there was an instance where we both had to kiss each other, albeit it was for a drinking game, but there were hard set rules of consent in place, meaning that he could have rejected kissing me if he really didn't want to- which he did not, but this could have been to be nice and not reject me in front of everyone. Also, the kiss was just a peck- no tongue. So, it wasn't really that big of a deal, but still. He also always invites me to hang out with him and his friends whenever they get together, to which I usually take the bait. He is probably just inviting me because he sees me as a good friend, but I can't help but question if there is more behind his invitations. Another important thing to mention is that there is about a 4-year age gap between the two of us (I'm 24 and he is 20- we are almost exactly 4 years apart, and our birthdays are coming up in May). I don't mind dating 4 years down, but I just don't know if he minds dating four years up, especially because he has a sister who is 25. I have a feeling that he might feel weird about dating someone who is nearly their sister's age. So my question for the guys is, have you ever hung out with girls with no ulterior motives? Is that normal for a guy to do? Another question for everyone is how can I show interest in him without seeming weird?

How can I tell if he is into me? How can I give him subtle signs that won't scare him away?
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