As your babies started holding their heads up, or at least keeping the…

As your babies started holding their heads up, or at least keeping their heads up for longer. At one month when I laid my son on his tummy, he seemed to lift his head, tried to hold it for a couple of seconds, but now just lies down, when you hold him with the face towards yours, the head wobbles in all directions, you could say that he doesn't really hold it, and if you turn his back to himself, he might hold it for a short while. I understand that all children develop differently, but everything is kind of worrying me, although they were born on time, they gain weight well, there are no deviations of any kind.