When he gets serious with you (IF he ever does) he will be able to overcome his, so called "urges" for sex and commit to you and only you. I doubt that you would deny him that if he came to you exclusively and it doesn't sound like an issue where you didn't please him enough in that way. Yes, he needs help and this help will only come from his own desire to change and commit to one person and stay with them. ONLY HE can decide this and nothing you can do to make this happen.

walk away. he isn't ready to change or commit and even if he is someday, is it really worth all the stress to get someone to stick to you and commit and stop playing around? Really when you find one that truly cares about you that will not be an issue ever.