Offer to masturbate with him, let him watch you do it, whatever you can think of to spice things up, really. It's not that your sex life isn't enough for him. Guys are going to masturbate, period. If he wants to do that more than have sex with you it's probably because it's easy, it's selfish, it's a personal thing. He doesn't have to impress himself, he doesn't have to be sexy or worry about who's cumming when. You just need to show him that he can feel that way with you too.
I am guessing the problem is he is not having sex with you that much and instead is masturbating, and this is making you feel unattractive and you are wondering why he does not want you like he used to.
As the above poster said, masturbation is normal, but only to a point. When you would rather masturbate than have sex with your partner, you are showing signs of addiction to masturbation.
The only way I see you being able to stop this in a positive manner is be upfront with him and let him know exactly how you feel. Honesty will be the best policy in this case. Your man needs to understand what he is doing to you and why he needs to check this before it gets out of control. However, you can leave "you have a problem out" this can cause him to become defensive and it will kinda halt progress. You should just tell him that you are feeling unattractive and less desirable because he cares to masturbate more than show you love. Ask him point blank, "since I know you love me, and care for me, will you please show me?" and do it in a nice tone.
Good luck, feel free to message me if you want to.
Masturbation is normal...