My kid was fine until he spent a month over his dads house he could use the restroom by himself and he was so active.Now he poops his pants and talks to himself normally and then in a whisper and also has burns on his cheek that were not there before.His speech is so broken he sounds like a robot. He can't even tell me what happens you know as far as explaining himself I don't know what they did to my son.Please help me?

I'd take my child to a pediatrician ASAP. Get a full examination and make sure any injuries are noted. The Dr. can help you get further care, like psychiatric help, if needed.

The doctor can also report injuries to child protection agencies, if there is cause to do so. What 'they did to him' isn't as important right now as loving him a lot and finding out if he is hurt, both physically and emotionally. And you may never find out exactly what went on.
How well did he know his father before you let him stay there for a month, and is 'they' the father's girlfriend, and does she have children there?

We can't tell you exactly why your child is acting differently then before. We can't see him. From your description 'talking like a robot to himself' appears that something happened to him emotionally that he couldn't fathom or handle mentally. You need to take him to the doctor; this does not sound good at all.

If this was happening to my child, I would not be sitting typing this information on a help desk and asking for help. I would be taking him directly to the doctor, or calling the police. You say that there appears to be 'burn marks' on his cheek which would indicate abuse of the worse kind. Take pictures to document this.

If he spent a month somewhere where there was any indication that something like this could happen, why would you leave him there?

Burn marks on his cheeks and you're asking us why he could be acting like this? Come on now....get him help!

I wouldn't wait for the pediatrician. I would have taken him straight to the ER when you noticed burns on his cheeks.

Is this really a serious question? Why are you delaying getting him some help by asking strangers over the internet rather than getting him professional help?

How long has he been back in your care?

I agree that this is very serious. Even if it has been several days, get him to a doctor and get him the help he needs.

Contact your/a lawyer and see what you need to do to legally keep him safe while an investigation into what happened is done.

Get him assessed! That you hadn't even mentioned that in your post, is strange.

THAT much noticeable 'damage' to your child makes you a part of this apparent abuse/injury, because you've done nothing to address it.

And why was he absent for a month from you? That's tough on a five year old.