When you say "milk", we don't know if you mean breast milk or baby formula. If you are giving him a bottle, he may be sucking so fast that he gets air in his tummy. All babies get SOME, but you can try to reduce that amount by breaking up the feeding into segments. You can try burping your baby in the middle of a feeding, then finishing the feeding. If you don't fill him up so much with milk and air at the same time, there is less spitting up. But some is normal. Also check the angle at which you are holding him.
A lot of babies get a type of newborn acne - bumps or rashes on the face. This often starts around 3-4 weeks and goes on for a few weeks or a month.
You may need to change formula but try these other things first.
And yes, call your pediatrician.