From my experience - masturbating and having sex with a partner are two very different things. Like you, I didn't see things in this way previously. I'm in a long term monogamous relationship now and similar to what you've described, we usually do it once a week (2 if there's a special occasion). I'd happily have it more, but that's what my partner likes and is comfortable with.

Really, the first thing to do is talk with him. Obviously at the start of a relationship ye have more sex, but as the relationship progresses - sex changes I think. At least from what I've experienced and have seen with my friends that are in longer relationships.

The important thing that I haven't seen mentioned in this thread yet is mutual respect. When you talk with him make sure you respect that his boundaries may be different from your boundaries, and talk frankly, but non judgmental.

He may masturbate once a day, but sex is something different. If you're both honest with each other then what's to come will be better (trust me on this one).

If you feel like your needs are not being met then that's the bigger issue. When I say that sex changes in a relationship, for me, it becomes much more intimate and more of an event. A really really fun, long event that you both share once a week. If what you need is different then you either need to compromise on what he can give you, or take a frank look at your relationship and what you want sexually.

One other word of advice - he needs to talk with you too - no good will come from you feeling like you're nagging him. You need to get him to agree to talk. This might be harder than you think (it ain't easy talking about sex!). Once he agrees to talk honestly and openly, then fruitful discussion can begin. If he's not willing to talk yet - you may need to work on that first and is something he's going to have to come to terms with doing.