My 1 yr old allergic to milk?

my baby is turning one this week. i stopped nursing about a month ago and tried switching to cows milk. but through out the day, he'd spit up.. it was only a little bit but it smelled like real vomit. the dr reccomended switching to soy milk. he's been on soy for about 3 weeks but he has HORRIBLE diapers. and it irritates his little behind. i just tried giving him cows milk again today but he spit up again. it was still only a drop but it stank like vomit. does this mean hes allergic to cows milk? should i try switching to rice milk?