Either reason, or another reason, could be why he doesn’t talk to you about it. :shrug:
Things you can do:
Pray often. As soon as you are tempted with the thoughts, PRAY. Don’t try to fight off the temptations alone. Pray to your Guardian Angel, to St. Michael the Archangel, to Mary, to a favorite or patron saint, or, of course, to Jesus.
Try to avoid any situation where you know you’ll be especially tempted. Take care of yourself, too, as Thinking10 mentioned, because being hungry, anxious, lonely, or tired can sometimes make it harder to fight temptation. If possible, get rid of and avoid anything that stirs up feelings of lust (certain books, movies, posters, music, conversations, etc.), especially in your bedroom. When you are tempted, you can also try doing something else, such as changing your position, environment, and/or thoughts to something that will help you discourage the desires to sin–or at least distract you from them. Maybe you can fold a load of laundry, or organize your desk, or call a friend to chat, or go for a short walk, or study for a test, or pray for your future husband, or whatever works.
Besides trying to avoid the sin, I’d also suggest that you work on cultivating the virtue (or virtues) that counters it. For lust and masturbation, those would be purity, chastity, and perhaps modesty.
Will pray for you!