I think that parents and step parents are allowed to have different opinions on returning adult children.
It's OK not to want an adult back.
I think a bit harsh when they're at uni, they haven't left yet. And when the travelling and new uni term means that this isn't even living with you, it's a visit.
Has he lived with your husband before?
If you have a one bed flat and your son has a history of disrespect to your home and your husband - I would see his point.
If your husband just doesn't fancy it... then tough luck, suck it up.
Don't ever let a man be a detriment to your relationship with your son!
If your husband loves you, then even if he's not keen (and that's fair enough if he's not) then he would say "why are you even asking permission? Of course he can stay! You know I'm glad to have our joke back afterwards, but he's your SON, he'll always be welcome here".
If your husband would not say that, for an 8 week stay for a uni student, then - I don't mean this flippantly - I'd be reconsidering my marriage.
No fucking way would my boyfriend be telling me my daughter couldn't visit!