Baby milk allergy?

So my son just recently turned one I have been slowing switching him to organic whole milk, he has been so constipated since We've started this. Some times he spits up a little chunky milk now and he has NEVER been a baby to spit up. Just now after four constipated diapers he had a huge diarrhea diaper.. not sure what to do his one year check up isn't for 2 weeks.. should I switch back to formula? Or try soy or lactose free milk? Advice please