Front fonatle not closed yet. Doctors?

My baby is 24months old (35weeks preterm) his front fontale not yet closed..when his teeth eruption was delayed when he is one year old we checked his vitamin D3 TSH and calcium levels and it all turened normal..and in 13th month his teeth erupted ..now when he is 24months old as his front fontale not closed we again checked his D3 tsh and calcium levels and as earlier his levels are normal infact he has 61 vitamin d3 level We consulted three doctors and all examined and told to wait so that fontale may close over time but we are still worried..his milestone were achieved normal in all stages ..without his preterm buffer time..he is very active child ..even though he is just 24months old he talks in sentences and started singing and buliding stories and shows a good reasoning ability... Whether we should wait for fontale to close normally or should consult other doctors?? We are really worried..