The only other thing that I can suggest is to put his litter boxes and trees where the dogs can't get to them (baby gate?). If he does not feel safe using his litter boxes then he won't use them! I can understand your frustration. I have had a peeing bengal and it's not funny. Can you let him in your bedroom(plastic dust sheets over the bed) but not the dogs and have a litter tray in there. He needs to know he has an area to call his own that the dogs cannot get to. I love dogs just as much but cats do need territory to call their own. It sounds like stress and he is trying to mix his scent with yours by peeing on your bed. Awful but it's because he loves you
If he can't become a happy cat then maybe a home with no other pets may help. Really feel for both of you and hope it can be sorted. Put a little bowl of biscuits on the sofa where he has peed as some think they won't pee where there is food.