Hi, I have read your question. First thing, (I am sure you must have already done it) is to get in touch with the doctor. Especially get your doctor to check him for colic. Some babies cannot get rid of stomach gas and get very upset. It gets very bad especially at night. My son had colic and stayed awake crying all night until my doctor gave a specific medicine for colic.
Secondly, if your doctor has given your baby a clean bill of health then the only thing can be that he is a "day-sleeper" and likes to stay awake at night. It might sound a little absurd to you but its true. Babies who stay asleep during the day want to get up at night and play with their mums (and dads) but since parents want to sleep the baby gets frustrated.
Its a traditional myth in some cultures that babies born during day time sleep during day time and stay awake at night while those born at night, sleep at night and stay awake during day. Its just a myth but for some it may hold true.
Hope this was helpful. :)