I agree with ches about the vitamin A, but I'm sure he'll be fine if it was just a tiny bit. Re the reaction, I just googled "chicken liver pate recipe" and the first 2 that came up had a large proportion of butter in, so that could've been it.
Good to know for me too in case I ever decided to give my 3 year old some, as he is also dairy allergic. I think I would've assumed it was just pureed liver and seasonings.
Mummy to two lovely boys - A born 27/1/09 and J born 9/3/11.