Brother's friend with gf seems to fancy me - how should I go about this?

One of my older guy friends (whom I like a lot but only as a platonic friend) whom I met through my brother last year literally won't stop staring at me every time we meet, despite the fact I don't see him around too much (I'm at uni though I graduate this summer and as we're in the same friend groups at home I'll be forced to see him all the time). He's a lovely chap but he's had a girlfriend for the past two years (though he has a tendency to flirt with a lot of other girls too and is quite a player; some of our mutual friends say he is "out of her league" so they egg him on to keep playing the field) and I have absolutely no feelings for him that way though I'd like to keep him as a friend. At first I thought I was just imagining things but he's always flirting with me (or sulks slightly when I don't look at him or talk to my closer guy friends!) and I've noticed he can't take his eyes off my face (I'm always spotting him looking at me out of the corner of my eye and then he jumps and looks away quickly when I catch him). Even my brother commented that he was "acting a bit weirdly" once when he saw him talk to me though they are close mates. I've just come back from uni for the Easter holidays and saw him today, he wasn't with his girlfriend and he asked how long I was staying for. When I said for all the holidays (till the start of May) he looked really happy and was like "That's awesome, hopefully I'll see you around at some point." He has always been there for me (he and my brother are both older than me), I just feel that the way he acts is inappropriate but I don't know how to call him out on it as strictly speaking he has said or done nothing wrong besides make me feel uncomfortable. I have a feeling he wouldn't keep doing favours for me if he didn't find it fun to flirt with me (he's really arrogant though nice to me) but it just makes me feel awkward. How should I deal with this? Don't wanna lose him as a friend/ruin the relationship between him and my brother but every time we see each other it's like his eyes are glued to me :S

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