Truth bomb incoming:
Your marriage is doomed.
So you want sex once a month, no oral, and you like it to last up to 20 minutes.
Like, of course he's miserable. You must be living in a self-absorbed whirlwind of motherhood if you can't see that.
You said in your original post, those 4 to 6 weeks can easily be 10 to 12. Probably even longer if he didn't mention it, am I right? That is not a sex life. That is having the least amount of sex with your husband that you possibly can. That is going to kill your marriage. He's telling you! He's telling you that it's eating him up. He's trying to work out nice, respectful ways to tell you without nagging.
Except... well, the nicey-nice approach doesn't work, does it? I'm sure you make him feel heard and valued when he talks to you about it. And then... nothing for weeks, when it will be up to him to drop hints and initiate as usual. Trust me, he will eventually lose patience and start nagging you. What else can he do, except ask a lot?
I guarantee you this will be consuming him. He will be thinking about this every single day. Probably as soon as he wakes up in the morning, and as he goes to sleep at night. Wondering when his next 20 poxy minutes might be. He will be resenting you deeply.
Do you care about that? Or in reality do you think he's actually got no right to feel that way, and he should simply shut up and be grateful for however much or little sex YOU decide he's getting.
If I could identify the single worst aspect of it as a man? Having no idea at all when the next time might be. Think about it, if you're eventually getting sex after weeks of NOTHING, you're going to want it to be as special as possible. (OK, well, your husband is). He's doing it now, you've told us. Abstaining from masturbation, probably shaving his balls etc. Perhaps extra attempts to set the scene, flowers etc? If you do all that, and then you don't get sex? Believe me, feeling "very frustrated and rejected" does not even start to cover it.
I don't understand why this isn't obvious, unless you really don't care about your husband.
He wants frequent, quality sex with an enthusiastic partner. Somebody who is sex positive, asks HIM for sex, is an active participant in your sex life, comes up with new ideas for the bedroom. This is a normal, healthy thing for a married couple to have.
Let me guess. The sex used to be great before the kids came along.
As you can probably tell, I have been where your husband is and it's fucking miserable.
This will get worse and worse over time, and I repeat what I said: your marriage is doomed. You have only two routes to marital happiness: either you somehow rediscover your libido, or he wakes up one day with erectile dysfunction.
Fingers crossed, eh?