Yes it is a thing. Or it certainly was for me. For a few years from my mid to late forties and into my fifties I'd noticed that my erections weren't as firm generally (although for some reason my occasional waking erections - "morning wood" - which have nothing to do with sexual arousal - were rock-solid, which at least reassured me that there weren't underlying health issues).
It was a gradual thing and I wasn't entirely sure whether it was to do with aging or tiredness from general life stress or a combination of both.
It was particularly noticeable when my wife was on top or when she was giving me oral sex. Sometimes I'd go completely soft halfway through and "fall out" which was very frustrating for both of us and I was worried in case she thought it had something to do with her or that I didn't fancy her any more, when actually I fancy her as much if not more than when we were first married nearly twenty five years ago.
Missionary position (and to extent sex from behind) was much less of a problem. I think a softer penis is maybe more easily "kept in" during missionary than when the woman is on top. In missionary - at least for me - the stress, worry and anxiety I increasingly was feeling about "keeping it up" when my wife was on top or during fellatio disappeared and I felt relaxed enough to become fully hard. I think that's where the positions thing comes in and what might well be happening with you and your DH.
There came a point when we began to avoid sex with my wife on top which really wasn't great because we both enjoyed that position and it's the one my wife orgasms in.
That's when I thought I'd give Viagra a go. And my goodness it made a total difference! It gave me very full, lasting erections and I found I could relax, knowing I wouldn't go soft halfway through in any position. Sex stopped being stressful and began to be enjoyable again.
I've since discovered Cialis (or rather a differently branded equivalent of Tadalafil) which has the benefit of lasting up to thirty six hours compared to Viagra's four, so sex doesn't need to be planned. Viagra and Cialis are both easily available online as well as over the counter.
Basically for me Viagra and Cialis have broken the spiral of anxiety which led to the problem in the first place.
I'd like to bet that what your DH is experiencing has nothing to do with how you are physically and you have no need to feel hurt.
Obviously I don't know your situation or how well your DH might respond to any suggestion that he might need "assistance". I can only say I'm glad I realized I did and found a solution that really made a difference.
I hope you can too and that you found this helpful.