Just out of interest, I found a government survey that suggests 73% of women call their mum once a week compared to 52% of men. So yes, not all men, but statistically contact is higher with girls. That's not an excuse for men and there's definitely factors that might affect this statistic, like style of upbringing, but isn't this part of what we're noticing when we see these trends? (and why it's important to notice and question them?).
I've had a variety of contact with my now deceased parent. I remember him getting very upset that I wasn't calling enough - to put it into context, he left the country and rarely called me - so we arranged for me to call him weekly. He then moved in with a girlfriend and needed less support/presence from me so then we were talking perhaps once a month or so (when I rang, unless it was a big calendar event but in his later years he was more forgetful so birthdays and Christmas didn't get as much attention from him). Sad to say I can't really remember what contact was like with my mother as it's so long ago now. I imagine we probably talked once a week or two when I was at uni (and my dad wouldn't have been that bothered to speak to me then as he had someone in the house with him).
I don't know OP, I feel like it's more than likely "out of sight, out of mind" rather than intentional avoidance - if you're happy you've got a good relationship. I don't see that there should be a problem with saying that you'd like to talk more and asking how he could see that working? Others on here have alluded to the fact that the relationship might not be as good as you think. I feel like, if you have a good relationship, this request should be discussed easily and matter of factly and he can suggest the frequency of contact that feels comfortable (even if means he tells you he thinks that the current set up is fine). If he feels like you've somehow put an unfair burden on him by mentioning it or decides to withdraw after that point, it might be sad but then at least you know where you are I suppose? If all the contact relies on you at the moment, you probably won't have lost much (would a simple request irk him enough that he would stop picking up your calls? Then I suppose it could be the relationship isn't as good as thought).
The thing about mother's day - it's a hard one. Likely a ND person isn't seeing the bigger picture here and might not see a reason to call. I find navigating all this kind of stuff hard. People always say that no one is responsible for someone else's happiness (I see a lot mentioned on MN that kids don't owe their parents anything and, usually in different kinds of posts, that parents don't owe their kids anything)... Which on the one hand I agree with, kind of.... But relationships would be weird things if the people in them didn't ever consider what's important to the other person and were only thinking about themselves all the time. Surely there's need for a bit of give and take?
I hope you find a balance that keeps you both content OP. Even if you don't mention anything, I'd keep calling rather than giving up. I know I'd love a call from anyone really. Even though he didn't give much, I miss having my dad to call just to keep someone up to date with stuff and to keep up to date with them. Not having anyone's regular updates and having noone to update now (at still a fairly young age) feels odd. Your son might be benefitting without really knowing at the moment.