Twin Development

Hi, I have 6 month old twin boys, born at 38 weeks. One was 5lb 11oz and came home straight away. The other 3lb 6oz and stayed in scbu for 2 weeks before coming home.

Please help...Am I driving myself crazy? The twin that came home with me is on the move. The other refuses to do belly time and just wants to lay there. His head control is nowhere near where it should be.

I'm so concerned about his lack of development. He has seen his ped and a physio who gave us exercises to do, but in the last month I have seen little improvement, but his brother is getting stronger. Could this be because he was smaller? Physio not saying a great deal so this not helping me with worrying that something could be wrong. Aplogies for long question.