My Son Is One Year Old And He Drinks While

My son is one year old and he drinks while milk . It seems as though as soon as we switched him he started throwing up a lot more , mostly with white chunks that smell of like cottage cheese. He also has a problem with constipation . his doctor prescribed a generic form of miralax twice a day , some days it will work and some days he'll poop solid round balls. Now today he keeps puking but its all clear . she doesn't seem to think he's lactose intolerant , or have any clue if there's something wrong with him. Any ideas?! Thank you so much!!