" DH adores his mum, much more so than his dad and feels guilty about that "
Well if his dad's a bully the it's not surprising that he thinks more of his mum!
And that's down to his dad.
Now, if it was just a question of finding one easier to get on with, I'd probably say "suck it up".
But his dad isn't just a bit hard to talk to, he's actually nasty such that his own son is scared of him, that's a different thing altogether.
There's no need for him to feel guilty at all.
In fact I would say that seeing him at all is way more than he deserves.
Let alone having him to stay for a week when he has been nasty to his DIL.
They don't need to see their GS, they want to.
Their GS certainly by all accounts won't get anything from seeing them.
It's sadly not going to make them care more for your husband or make him any less scared of his father.