baby fighting to sleep

Hi mummies any one out there having baby that fight to sleep? Im having 3months baby that is always fighting to sleep. I can see he is already sleepy but still try hard not to sleep and will cry murder. Already take note his diapers milk and everything is ok. Adjust the room temperature and noise level but still the same. Will everything change back to normal cause he is not like this when in newborn stage.Why is that so?? I need HELP!!!