Thanks for all your help!
> We're not sexually active and don't plan on becoming sexually active
> until we're married (if we get there)
OK, that explains some things. If it is so, then it is quite natural for him to masturbate.
> ...though I'm glad you found it be effortless.
Well, to put things into perspective, for me it was completely effortless since I had contact with my then girlfriend at daily basis. But even with less frequent contacts later on, it was not difficult to avoid masturbating. Once a man "tastes" a woman, in general he does not look back. There are exceptions of course.
> I know it's just a matter of will power, but do you have any
> suggestions on things that may distract him if he gets this urge?
I don't see why avoid to masturbate if you don't have intimate relations. Even if he manages to completely abstain, he may wet the bed after some time during sleep, especially if he is still very young.
If for some reason you wish distraction as a help to abstain, then this depends on what your boyfriend finds interesting. As a general rule, he can practice hobbies that are either energy consuming, like some sport, or demanding intellectually, like puzzles, reading/studying, learning and practicing music etc. But above all, it must be something that can absorb him. Only you two know what this could be. And of course the outcome will also much depend on his will power. Oh, and don't underestimate the power of the subconscious, it will do its job during sleep.
We're not sexually active and don't plan on becoming sexually active until we're married (if we get there). I guess for a lot of men it's hard to stop masturbating at all, though I'm glad you found it be effortless. He can go about 3-4 weeks without doing it and then seems to cave in. I know it's just a matter of will power, but do you have any suggestions on things that may distract him if he gets this urge?
Well, what's the problem here? Your boyfriend has apparently a girlfriend (you) and he is trying to stop masturbating. I see nothing wrong with that. When I had a girlfriend I needed not even to think about stopping masturbation. It happened naturally. All my energy was for the girl which I loved so much. It's that simple.
I am not sure if I answered your question; the answer is so straight that I am wondering if I understand correctly.