But I believe in his mind, that’s what he believes. That if we get back together he will be happy.
i don't think his goal is to be "happy" - i think his motive is to just be let back in the house and not have to deal with being responsible to himself and his own decision making (or lack thereof) and not have anyone to run around and fix everything for him or someone else to blame and share his misery. he wants his status quo back.
best to stay out of his head and what you think might be going on in there. stick to your own head. what YOU think. what YOU want. you absolutely deserve to be happy in whatever way makes you happy (minus anything classified as a felony!!). you are not OBLIGATED to stay miserable and wait for him. the universe will appreciate your joy.