Hello there, tooth has already weakened and damaged, this requires immediate attention kindly do visit a pediatric dentist and according to the depth of the cavity the treatment plan will be explained. Front teeth will be replaced in the age range of 6 to 8.. Now the kid is growing and tooth cavity has infection which is not good for the growing child. Losing front tooth also affects the speech in a major way. Proper oral hygiene has to be maintained and night time milk feeding is the main problem for this type of cases.

It mustve broken either due to decay or weak teeth. To find the problem,  you would need to atleast post a picture though it would never replace a clinical examination by a dentist.  Do show him to a pedodontist.  It may not be linked to his premature birth.

Show him to a dentist. If you want to consult on practo then better to do a video consult inorder to understand what is the exact problem. Treatment is case specific.