OK, you do accept that the pair of them moving in with bits and bobs and pets and so forth is not going to happen right? It's how you go about telling him that's the problem for you, since DH has washed his hands of it, he is SS to him I think?
Anyway first thing, how are they going to save if they give up their jobs?Delusional.
If I were in your position, I think I'd probably offer the following scenario (because he is your son).
Sit him down and read him the riot act about not consulting you. Start as you mean to go on.
Then offer HIM a spot to sleep in - no pets, no furniture or effects. His girlfriend must stay with her parents, sibling, friend whatever, until they have saved enough to move on.
That way no one is homeless, but they might be motivated to get with the program and save like mad to be together again.
Just a thought, and only if you think they will agree. If they insist on being together with all their detritus and pets, refuse point blank.
Tough one in a way. Mums will be mums!