My son is 4 months he won't keep his milk down what should i do? he's been doing fine on this milk before until now

Depends upon...: ...How long the spitting up has been going on. If it started suddenly then more likely to be a viral infection which should pass. The key is to keep baby hydrated. Smaller, more frequent feedings of regular breast milk or formula or full-strength pedialyte (or store brand of it) is best. If baby looks very ill appearing or cannot make a wet diaper every eight hours then should be seen asap.