It really depends on what kind of relationship you currently have with him. You’d have to give me a few more details if you’d like a more in-depth opinion. For now, all I can suggest is that you move slow in how you communicate your feelings. If you’ve felt this way for a long time and you really want to see if there’s something there, try to let him know how you feel. It’s better in person because this way you can keep your privacy and there won’t be any texts or written stuff that anyone might use against you.
If having relations with your cousin (step cousin or biological cousin) is legal where you live, there shouldn’t be any issues. But if not, be mindful of how open you are.
The age gap is not important if you both don’t feel that it is. But if he is under 18, you should wait till he’s reached that age so you’re both mature enough to do anything sexual and it won’t become a legal issue.
Since you’re not blood-related, you don’t have to think about reproductive risks specific to related people but it wouldn’t hurt to think of that stuff in a general sense (as applies to non-related people) if you ever do get to that point.
For now, I suggest you think about those factors and IF you two are of legal age, it’s up to you how you proceed. If one of you is under 18, you could still share feelings, but to be on the safe side it’s better to wait till you are both 18 or above to do anything more. Be prepared for any response – take a chance and tell him, alone and in-person, when you are ready.