This is probably not something that can be answered without more information. But I'll do my best -
First of all, you need to establish why you find the need to be clingy. Is it because he doesn't make you feel important or like a priority in his life, or is it because of your own personal issues?
If it is the former, perhaps communicate this to him about how he makes you feel and why. Then you can maybe start working on these issues together.
if its the latter, then maybe you need to reflect on why it is you feel that way. Could it be that you feel needy or insecure when he's not around? If so, it would be helpful to do some personal development - find your own hobbies and interests and keep yourself busy, so that you can work on being more independent and and happy when he's doing his own thing. That way, you'll be wanting to see him when you want to, not because you need to. A degree of independence is a very attractive trait in a partner. Then he will be the one wanting to spend all his time with you!
Hope this helps :)