I can't tell you which it is because it could honestly be either. Only you can determine that for yourself by looking deep within yourself and thinking about it. Don't be afraid of spending some time by yourself to reflect on this. I feel like everyone should do this more often....
By nature, I'm a pretty possessive person. I have toned it down quite a bit over the years but I still get those urges so I know how you feel. When I get to the stage where I think that it's pretty unhealthy to feel this way, I start to think what could be the source of this feeling. I try to think if I feel anything special with them or if it's just my lonely side trying to hold on to someone. The key is being brutally honest with yourself. Don't allow yourself to brush off things. Take the time to think about it.
By the way, having a crush on someone does not always mean being sexually attracted to them. I am typically not sexually attracted to males and yet I have crushes on them all the time because I enjoy being in their company, I like their personality, they make me feel special, and/or I find them aesthetically appealing. I'm more sexually attracted to females but my crushes on them are fairly selective because I am honestly intimidated by females who I find sexually attractive and thus I don't really know them very well. My point is that even though you aren't sexually attracted to your friend, there is still a chance that you have a crush on him for other reasons and that is perfectly fine.
Well whatever the reason is behind your possessive behavior, you probably know this by now but it should be toned down. Feeling this way, whether it is for a crush or just a friend, is not healthy. For one, this person has the power to severely disappoint you multiple times and you should never want to give someone that power. You will be prone to being hurt and suffer a great deal for no good reason. Another reason is that most people do not like those who behave in a clingy manner. The more you try to hold, the more they want to get away. However keep in mind that just because you don't want to come off as clingy, you don't have to be okay with every time they blow you off or something. You still have a right to say that you don't like it when they're "too busy" for you, just don't be overly dramatic about it. In other words, try finding a balance.
Anyway, I hope this helps. Please don't hesitate to ask more questions!
