My son does not want to eat food,since yesterday and also threw up his whole milk.tried giving him food and says no. what could it be? he is 1 yr old.

Recommend patience.: By this age, not eating for a couple of days or more is usually very low risk. The behavior could be related to any number of emotional (any current family upsets?) or GI upset issues. Pushing food he does not want is only likely to create problems with eating & food long term; not a good idea. Better to be patient, listen for clues, rely on his built in desire(s) to do what is better for him.

Illness vs behavior: You can discuss with your pediatrician. There can be underlying illnesses that can cause this or this can be behavioral. Your physicians can help clarify which of these are happening. The vomiting may suggest an underlying illness like gastroenteritis. This needs to be clarified by the pediatrician.

Needs to be checked: It is very difficult to say what is wrong with a 1 year old who is refusing to eat He needs to be checked.It may be he is teething or may have an infection Meanwhile make sure he drinks fluid and if he throws up even fluids,he may need to be examined asap You can try clear liquids first