mom help

So I have a question/concern, my son just turned six months old on the 21st he was born two months early so technically is four months, however in some developmental ways he is on target with a six month old....however when it comes to tummy time he isn't even on par with a four month old....his foster mom says she does tummy time with him and that he just doesn't like it. I agree he doesn't like it but I still have him do it for as long as he can tolerate it...I'm just concerned about how he just lays there he doesn't try to hold his head up or anything he just lays there...I don't know what to do and I'm worried.