Yes, I think it is normal and it does not have to lead to any worries or problems IF he does not refuse to have sex with you, in other words, if he does not ignore your sexual needs by prefering masturbation to having sex with you.
Some of the reasons why he masturbates can be, for example,:
-if he wants to have more sex (more often) as he can have with you, then masturbation can be his way how to deal with the fact that he needs more sexual activity than he can get
-if he can have sex with you whenever he wants and he still masturbates, it can just mean that masturbation can offer him different kinds of experience (e.g. he can focus only on himself, his reactions without having to modify his activity according to the needs of the partner who is having sex with him)
-if he uses porn during masturbation, it can give him another dimension which he cannot find in real life sex
-he can masturbate in situations when he feels sexual urges and there are not suitable opportunities to satisfy those urges by having sex with you (e.g. time, place, situation etc...)
In sum, I would say that if he uses masturbation just as an ADDITION to his sex life with you it is quite harmless and normal. The problem would start if he used masturbation mainly as a SUBSTITUTE for real sex with you (which may lead to your as well as his sexual frustration in the end).
men masterbate. some women do as well. if he's not cheating... i wouldn't worry about it. if it does bother you...talk to him about it.