It sounds like this is really starting to stress you out. Maybe the best thing is to ask the parents to find other care.

It's ok to tune out once in a while but if you really feel that you re having a hard time caring, maybe this isn't the spot for him. Perhaps the two of you just aren't bonding?

You can't deal with this forever and as you said it is really affecting the other children. That is not ok either. The more life he sucks out of you the less you have to deal with the children that are doing well. Those kids are probably missing out on a lot of attention while you try to deal with the screamer.