Dog bigger than expected

I have a what was supposed to be miniature cockapoo but he will not be miniature at all. I love him to death. However when we got him he was supposed to be 11 months he is 5. He was supposed to be small he is almost 17lbs now! Me and my boyfriend live in a townhome with no yard. We love Walter to death but are worried about not being able to take care of him properly with him expected to be upwards to 45lbs dog. What do I do? I feel bad for rehoming or surrendering. I would love to keep him but he will just be too big for the space we have to offer. I’m at a loss cuz I was super excited for baby Walt to join the family.