Lifting head up when doing tummy time

When did your little one start lifting head during tummy time my son is 2m4d and shows no sign of being able to lift his head he just lays flat sucks his hands or the blanket even if he’s not hungry, I hold his arms in but still doesn’t lift it. Any tips?? Suggestions?? I’ve just seen babies his age being able to hold head up pushing up with their arms and everything and he’s not even close, he was born 3 weeks early idk if that could have anything to do with it. I just don’t want him to be behind and I don’t want his head to get too flat since he doesn’t like to be in his belly. He does hold his head up really food when I’m holding or holding him sitting up with his arms he looks around and everything