The first thing you need to understand about male masturbation habbits is that pornography is not an 'instead of', it's an 'as well as.
Although you may feel rejected because your personal sexual activity is low, it doesn't mean he prefers masturbation to sex with you - it's simpy that it's easy, quick, and takes no thought on his part, which makes it attractive to almost all men.
If you really feel he is prefering that to sex with you, then you need to discuss it, but go carefully.
All men grow up with the notion that masturbation is a 'guilty pleasure', so it is something he won't want to discuss specifically, so don't ask for details.
Approach this from another angle - try instigating some sex yourself, and don't be shy!
You may find that taking control is something you both enjoy, so take a deep breath and see what happens - be prepared to be patient though, he may have just got out of the habit, and alpsed into simple self-satisfaction because it's easier.
You may not stop him enjoying pornography, but if you are secure in your own sex life with him, you can see it for what it is - a simple 'add-on' when he feels the need.
Good luck.