So many opinions
This is a very broad area. A man in his 20s may want to have orgasms multiple times a day and there is absolutely nothing wrong with that. Erotic movies stimulate men and there is nothing wrong with that.
People have levied some VERY charged opinions here without knowing some basic info. 1) WHY do you have sex once a week? Do you ever initiate? Does he reject you? 2) Is it possible he is having some confidence issues? Happens.
Sober people have this weird tendency to over-analyze EVERYTHING and try to relate it back to addiction. Masturbation is a gift from God. And if you and your partner are truly in love, relationships become marriages when they work AROUND these things, not make them deal-killers. If it gives him pleasure and does not take away from your sex life (and you have not drawn that link one way or another...others have done so for you on this board), then really, who cares? I mean you do. But you are not in his skin. Bet you anything he would not mind if you masturbated next to him.
You two are young. Your tastes may be different but if you love each other you can learn to appreciate, dare I say it, a porn movie with him from time to time. And he can take it easy and try to show some more discretion. but if you send him into hiding on this, there will be a bad end for sure. He's not selling drugs or shooting up schools....he's giving himself pleasure. Give him, and yourself, a break. Take it easy.