M.P.
If you do not have a legal document giving you custody/guardianship, I strongly advise you to do this RAW. Along with the document comes the legal right to control who he sees and when he sees them. If you cannot afford an attorney call Legal Aid. Getting legal custody would have been easier when the parent(s) were not involved. However, the long period of absences will be taken into consideration. The court will order a home study and back ground history if the mother wants to have custody of the boy.
Another option is to talk with a Juvenile Court Intake Counselor. This situation is not good for your boy. They may have some helpful suggestions to help you gain full custody.
Whether or not he sees your body or his mothers has no bearing on where he lives. If you want more privacy then explain that to him and tell him not to come into the bathroom. When he does come in, remember it's a habit and will take time to break, ask him to leave. Do not make seeing a woman's body a big deal.
He is a normal boy. Please do not be concerned about his preoccupation with boobs. My grandchildren, a girl and a boy, age 9 and 6, are still interested in my breasts. They like to cuddle up on them and pat them and say, "squisy." Breasts are our first source of nutrition and comfort. I do teach them that touching them is not appropriate in public and I'm working on getting them to stop at home too. But....I'm not worried or concerned. This is just part of growing up. Your 5 yo has the added interest started by his mother's pregnancy and body changes.
I strongly urge you to get security for him in his life. You need to have legal custody and legal boundaries for his parents involvement. I would be more concerned about the disruptions his parents' on again, off again way that they show up in his life.
I know that his interests in girls and boobs is caused by his mother's pregnancy and his uncertainty about what is going on. His mother gave birth to him. This new baby will suckle at his mother's breasts. Did he? He may want to now. A pregnancy and baby bring up all sorts of issues from the past. For him most of those issues happened before he had a language with which to describe them. So he's going to act them out rather than talk about them.
I suggest that you talk with a child psychologist and perhaps also get your boy involved in some child therapy. He has too much going on in his life for a boy his age.
Do not allow his interest in the female body take you away from paying attention to the primary and most important issue of a secure living arrangement. You should be concerned about the body issues IF you think that the mother, her boyfriend, and/or the children are doing sexual things with him. If that is a possibility do not leave him alone with any of them and make note of anything that seems unhealthy to you.