Doesn't look like anyone above has used the term boomerang, but that's the vocabulary. Get used to it.

Suggestions above are all good. Written formalities help to avoid confusions and confrontations.

Biggest challenge in these situaitons is getting past the historical roles of parent/child and shifting to four adults all sharing a property, with only occasional reversions, as appropraite, into parent-child modes.

The good news is that when this works it is a wonderful opportunity for building an adult-to-adult relationship with your child and his partner.

And remember, too, that this will create potential issues with the other two sons and their families, who are not part of the new domestic intimacy.

Good luck! And enjoy your son's cooking.