I forgot to add this earlier: your boyfriend might be secretly dealing with sexual dysfunction, or maybe even a porn addiction? You can do research online to see if any of these descriptions match any signs that you have noticed about your boyfriend's behavior.
Hi,
I'm sorry you're having to deal with this. In my opinion, masturbation is perfectly normal, unless it takes precedence over an intimate relationship with your significant other. My suggestion to you would be to have one more open discussion with him about how this affects you. Be gentle, but firm when you tell him that his masturbating should not take priority over sexual intimacy with you. Tell him once more about your expectations you have for the sex life that you want to share with him. If he is non-responsive, or if he shuts you down, this is a definite sign that you might not be compatible. If you don't begin to see positive changes soon, don't waste any more time on this relationship. Everyone deserves to feel appreciated and attractive when they're in a LTR; if that is missing in this relationship, consider moving on.
Good Luck