Masturbation is normal and healthy as long as it isn't excessive, and as long as it isn't interfering with the man's relationship or ability to perform sexually.
If he's masturbating say once or twice a day, it's totally normal and I would advise you to either try to accept it or try not to make such a big deal out of it. If he's just engaging in a normal amount of masturbation and you just have a hang up about it, it's YOUR pressuring of him and making it into a big deal that is causing the problems. In that case, you're making him feel guilty about something that is totally harmless. Especially if this opinion of yours is something that has developed (or increased) as a result of you finding the Lord, that's kind of unfair in a way. You would be projecting your beliefs onto him.
Now, if he's doing it excessively, is obsessed with it, or it is interfering with his ability to be intimate, then yes, it's a problem, and I agree with seeking out a couple's therapist, maybe a sex therapist to work on these issues.
Also, I have to say that while it's wonderful that you found the Lord and have decided to be abstinent, try to be a little understanding and compassionate that that's a big change for him to accept, if you were regularly having sex, to nothing at all. It sounds like he respects and honors your decision, but keep in mind that that is a pretty big adjustment.
I guess I kind of also don't get religiously or spiritually how that impacts a person. I can see if a person was a virgin and has decided to wait, but for someone who was already sexually active, that just doesn't make a lot of sense to me. Is it a guilt thing? Truly, I'm trying to understand. IMO, you can have a relationship with the Lord and your faith without your sex life having to be involved. I just don't think the two "go together" per se, not when a person was already active. Just my two cents. It's not like you can erase or "undo" your past sexual history and become a virgin again.
When are you getting married btw? (Congrats on the engagement). Best of luck to you!
From a male perspective, having sex or not, men will still masturbate. There are many reasons why. Often times its an escape from every day stress or depending on his sperm build up needs relief. Hormons also play a part and the younger the male is the more he will do it.
I would not bring this subject into the relationship as an issue of the troubling sort. Its natural and often times necessary.
Once a person has experienced an orgasm self induced, doing it again will always be tempting.
To me its not a sign of a problem with your relationship or his love for you, its merely a fact of life.
I'm not sure what you can really do for him, but to be understanding of the situation, but if he is doing it often enough and feels there's an actual problem, he should see his doctor, and possibly a therapist (not saying he's got a mental disorder), but a therapist could possibly help with an underlying problem that may exist causing him to feel the need to masturbate often. If it's only once or twice a day, he may not really even have a problem, but if he feels he does, he should seek out help as there could be something going on internally that would cause him to be focused on doing it.
This is a common issue with men who have just stopped having sex. They remember the feeling of what it felt like and crave it. Although masturbation is normal and some will argue that it's actually healthy, it can and will more than likely cause problems in a relationship. Maybe you both could try seeing a sex therapist or a relationship counselor to help or possibly find other things to do to keep the both of you busy with other things, find new hobbies, etc. It might take his mind away from the craving.