I have to agree that saying "My child can't live with his parents, but I can live with mine" is hypocritical. I see your husband and son as two ultimately different problems that happen to be tangled up.
Is there some history that would cause this response in your husband? Does he have a sibling or close relative who's lazy and mooches off of everyone (I do and I know a few people who do)? Does he just really hate your son's choice of girlfriend? Did the two never get along? Did your son make a career choice your husband doesn't agree with? Unless there's some background to explain it, your husband's actions are pretty concerning, especially because you still have so many kids in the house.
Why would your husband have to bite his tongue? This is his child, albeit an adult one, he should be able to live with him without constantly sniping. He should be able to offer guidance without it turning into mental abuse. If your husband has legitimate concerns about how your son is living his life, I don't think it'd be uncalled for for him to talk to your son about it, but he should be able to do it in a way that isn't overly harsh. I definitely value independence, but we can't forget that in a lot of societies it's common for families to be tight-knit and for multiple generations to live together. Parents and children should be able to live together, even if it's not ideal.
At the same time, while your husband sounds quite out of line, he also may have a bit of a point. You may want to take a big step back, if you can, and very objectively look at the situation. Is your son lazy? Is there reason to believe that, if you let him and his girlfriend move in, he'll be able to take advantage of it and you'll just be enabling bad behavior? It is possible, and you do want to be careful of that.
If that's a real concern, or even if it isn't, I'd suggest a few things: First, consider if the girlfriend truly needs to move in with you, or if she could stay with family/friends. I imagine living with you and 8 kids may not be pleasant for her, and it'd also give your son more incentive to get back on his feet. (There's also the risk that the girlfriend is part of the problem, so separating them may help him more than you realize) Second, set up some sort of rent and expenses agreement. I realize he just lost his job, but it's not a bad idea to hold him responsible for his own expenses. Again, incentive to get back on his feet.
I have a BIL who objectively is lazy and is making bad life choices, in no small part because MIL enables him. He's a grown man, and she acts as if he can't handle any part of his life himself- she doesn't allow him to take responsibility for his own life, so he never does. Part of hte problem also is his girlfriend. I think she's getting better now- but for awhile, she was seriously hurting his ability to get and keep a job because she wanted him to be by her side 24/7 and would emotionally manipulate him into never leaving the house. He couldn't job search, he blew off work when he had it, etc. MIL let them both live with her, rent-free, for over a year and a half and they've now moved back in. I have no idea if this could happen with you, but I have seen it happen and I can understand your husband's fear of it even though I don't agree with his attitude.
this is just a moment in time, step aside and let it happen