What can cause extreme tattoo swelling?

My fiance got a new tattoo yesterday. It's a huge tattoo, covering his whole forearm and it's very colorful (he has about 8 other tattoos, all black and gray). Everything was fine, he was taking care of it like he's suppose to and about two hours ago it started swelling extremely bad. I have done some research and found that red ink and green ink can cause allergic reactions and the majority of that tattoo is red and green. I gave him a allergy pill in hopes of the swelling going down because he had to leave for work. I was wondering if anyone has experienced the same thing? Should he see a doctor or wait a few more days to see if the swelling subsides? And what could the doctors do?