I am not sure whether YABU or not yet.
Dp had this situation literally a month ago, no qualifications except those relevant to his current (now previous) job, but he was desperately unhappy, because of several issues illegal conduct at work.
He wanted a change. We both knew if he retrained in a new industry money would be less, things would be tight. I'm on maternity leave so when I say tight I mean tight.
But, I would rather him be happy, relaxed at home, actually enjoying his job, than have a spare couple of hundred quid each month.
He got a new job in a new industry and loves it, hours are better, there's overtime if he wants it, a lot of chances of training for different equipment etc which every time he completes one his wage goes up. Will be 100 times better when I go back to work.
It's worked out for us, he's worked his arse off doing overtime etc and were not that much worse off, but most importantly he is happy. Which means we argue less, he isn't stressed, he doesn't get home from work and sit in silence all night.
It's made a massive difference to us, and if you're confident your DH would make it work, then I think you should let him do it, but obviously only if you can cover all your costs in the short term while he retrains.
I don't honestly believe you can stay in a job you hate long term, it's utterly soul destroying.