Okay, I am going to "assume" something because you didn't say so here? Did you just "stop" giving him formula and go cold turkey to milk? If so STOP. His little body has to adjust to milk. I have years and years in Child Care and I can tell you that when you stop formula one day and start milk the next you are tearing up his entire system. His body is telling you he has a problem.
I definantly say you need to get a call in to your peditrician and discuss this with them now, this morning. If it is a milk allergy then you have to deal with the allergy part immed.
I can tell you only of my experiences with children and transitioning them from formula to milk. Most parents have, when the baby is at about 11 months old, started 1/4 milk to 3/4 formula for a week or so, then 1/2 milk to 1/2 for another week or so, then 3/4 milk to 1/4 formula. By the time they are a year old they are taking mostly milk but this gives their tummies time to adjust. When I have hade a baby room parent just change overnight that baby was screaming all day for weeks, the pooh would turn white and stink so bad that teachers in another room would puke when that baby's poopy diaper was changed. Literally puke. The baby couldn't eat, they eventually got over it but it was way harsh on their little bodies.
There are lots of alternatives to milk, there is formula for the older baby, we received WIC for all of my grandkids and WIC will even cover extended use of formula if your doctor writes a prescription. There is no law that says your child has to go off formula by age 1. Many moms here have suggested different options too. Discuss it with the pediatrician and do what's best for your child.