Worried my boy is behind on his milestones

At 4 months his tummy time has regressed. It's been bad for a month now but I keep at it. He favors one side when rolling over and it seems like the other side of his neck is tight. I switch up tummy time all day to keep it interesting but he just cries. I'm worried about his neck and I've noticed flat spots on each side even though I try so hard to keep him off his head during wake time. I don't know what else to do to help him enjoy his tummy. He'll be 5 months November 3rd