Have you tried lying there, having a vagina and...oh, that's all men need. Are you sure you aren't squirming around too much, making it hard for him to aim?
Look, I don't mean to be crude, but I assume you've tried putting your husband's penis inside your mouth, because that's the most natural of all sexual acts. So I won't suggest that. By the way, in case I'm being unclear, if you aren't blowing your husband, that's your problem right there. Mystery solved.
Henceforth I will assume that you are A) lying still 2) in possession of a vagina and *) capable of making blowies.
There's only one thing left, and you're not gonna like it...
I don't think I need to get into graphic detail, here, but it is possible that your husband has grown a little bored with the 'regular arrangement' and fantasizes about jamming his heated flesh puppet into your recently gravid corn hole. This is not as bad as I'm trying to make it sound. You just have to go slow - fingers first - and make sure you stay as relaxed as you can. Oh, and tell him not to rage in there. You gotta take the butt stuff slow or something's gonna rip.
If regular multiple position sex and blowjobs aren't making him finish, maybe something is actually wrong. Like he's sick, or he's in love with another woman/man. Maybe his job is destroying his ability to be happy, or maybe you nag at him all the time about stupid shit. It could be built up feelings of regret, shame, guilt; or a lack of depth in his life because he feels like he hasn't provided properly for you, or been as available emotionally as you'd like him to be.
The butt sex will cure all of this.
Short Answer: I repeat. All of this.