Legal Question in Family Law in Massachusetts

You're not really going to get a straight answer here; there are too many variables.

You may not know much about the boyfriend, but the mom may know plenty. The age of the child will certainly be an issue. As will how often the child sleep/slept in the bed with them. When it comes down to it, just because something may be weird doesn't make it wrong.

That having been said, you are being a good parent by being concerned. I certainly would be. But without evidence of wrongdoing (or without reason to have belief of wrongdoing), it may be hard to get a court to take any action. But that doesn't mean you cannot have an open discussion with the mom. If that doesn't prove possible, you can always hire a PI to see who this guy is.

And if you do have reason to believe the guy isn't on the up-and-up, or if the child is old enough that co-sleeping is not socially acceptable, or if the sleeping arrangement is more or less permanent when it shouldn't be, you can petition the court to change the custody arrangement to best protect the interests of the child.

There are so many factors to be considered in this question, that it's impossible to answer accurately. You would need to provide much more information, such as the age of the child, whether the child wants to sleep there, what you know about that boyfriend, etc.