It is possible for an 18 month old to become diabetic but it is certainly uncommon. There are other reasons for increased fluid intake as well, such as water regulation problems. The difficulty is figuring out if his increased urination is because he drinks so much fluids or if he drinks so much fluids because he is trying to compensate for the urination (sort of a chicken and egg issue).
I would suggest you have your son evaluated by his doctor. Some screening labs, such as a urinalysis are probably a good idea.
Advice and opinions given are based on the information provided and must not be considered an official medical recommendation. The limitations of the internet prevent an appropriate evaluation. Always consult your doctor if there is concern.