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I am not sure if I answered your question; the answer is so straight that I am wondering if I understand correctly.
> 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.