More than marking, vindictive peeing

We rescued a pair of brothers when they were 2.5. Put them in separate kennels at night helped behavior issues. One has had separation issues from the get go so we kennel at night and when we leave the house. Now we are fighting with an increase in what I can only describe as vindictive peeing by the same dog. He pees on the chair in my son’s room (baby came after dogs, kid is now 3.5 and still having it happen occasionally) and has now fully peed (not just a quick mark, a puddle) on three locations of my bed skirt. He is wearing a diaper right now for behavior modification. Put it on in front of the spot he peed on.
How do we get it to stop so our house doesn’t smell like pee?!