Given that you've been married 20 years OP, I'm going to guess you and your partner are early 40s at the youngest?
I'm male, and unfortunately have not been blessed with a particularly reliable set of equipment for reasons unrelated to age, which means that I've done an awful lot of reading on the subject over the years. I also had an enlightening but TMI conversation down the pub with some friends a while back.
To go into graphic detail. Basically the penis has a sliding scale from flaccid -> semi -> hard. And visually, that's the end of the scale. But it's not, it keeps going. There are degrees of hardness beyond what most women, when looking at or touching it,would consider hard. It's more of an internal feeling, that our penis is throbbing.
As we age, that far end of the scale becomes harder to reach, and we more often have to settle for hard, rather than throbbing. Which is fine for actually doing the job, except that without external stimulus, we can't actually tell from internal feel alone whether we're hard, or losing our erection. Without the throbbing, we don't really know if we're hard.
So as we age, pretty much every man seems to through a crisis of confidence where they're constantly afraid during sex that they're losing their erection, until they come to realise that this is the new normal. I've not had it, or rather I dealt with it back in my early 20s when I realised that it didn't always do what I told it to.
But every other man in that conversation in the pub was either going through it or had come out the other side.
And along with the inability to reach "throbbing", also comes the inability to orgasm. Partly because it's harder to give over to the moment when you're constantly thinking "Shit, am I going soft", partly because most of us just aren't as fit in our 40s as we once were, and partly because without that throbbing feeling, it's just plain more difficult.
And most men generally find blowjobs / hand jobs easier to orgasm from, because they're less likely to be doing all the work, and they involve a lot more friction than PIV. So when we start struggling to orgasm, it's generally during PIV.
So no OP, your husbands issues are unlikely to have anything to do with you, or any changes to your body. Instead, it's all about the changes to his body, and the mental ramifications of that.
Sorry if any of that was TMI, and thank you for listening to my TED talk!