T.N.
My first reaction is...Why...? Why are his clothes wet? Why is he not wearing the alternate set of clothes left for him? Why is his nose bloody (What happened)? What happened to cause the bumps and bruises?
If these questions have not been asked and answered satisfactorily, that is a good place to begin, particularly if this has happened more than once. Also, is your current boyfriend the biological or adoptive father of your son? If not (and he is not your husband), neither he nor his parents have ANY say in anything to do with your son. If he is, he may not agree with your feelings. However, he must respect those feelings.
Not to scare you, but if you feel your child is being neglected or endangered in ANY way while at your boyfriend's parent's house and you choose to leave him in that situation, you could face CPS, charges or worse. PLEASE ask the tough questions and look for answers that make you feel at ease. You can ask these questions without being aggressive or accusatory. Be assertive in asking and go with your gut feelings on the answers received. Worst case scenario, it is dangerous and you need to escape the entire situation with your son. Best case scenario, your son is rambunctious.
Good luck! If you need to talk, please feel free to send me a message.