Feeding Liver

[Skye2]

I have a very fussy eater. Two of my dogs eat anything but my bulldog won't. I feed royal canin with some meat. The only two things he loves are chicken necks and cooked liver (either chicken, lamb or beef). I cook the liver cause he won't eat raw. I cook it in the microwave with water and pour the gravy over the kibble and give him a small amount of liver. I read somewhere he should have liver only once a week. I was wondering if this is correct. He likes lamb hearts also but again only cooked which is a good alternative.

My question is what is a safe level of liver to feed and would cooking it destroy the vitamin A content so he doesn't get overloaded with this.