So I was able to dig up one article out of a Professor of Social Anthropology at the University of Sussex. I think this subject is going to be hard to find more information on due to Idi Amin having lived in contemporary times.

The article quotes one of Amin's cabinet minister's book, Henry Kyemba's State of Blood, as saying this:

Amin’s bizarre behaviour [ … ] derives partly from his tribal background. Like many other warrior societies, the Kakwa, Amin’s tribe, are known to have practised blood rituals on slain enemies. These involve cutting a piece of flesh from the body to subdue the dead man’s spirit or tasting the victim’s blood to render the spirit harmless [ … ]. Such rituals still exist among the Kakwa. [ … ] Amin’s practices do not stop at tasting blood: on several occasions he has boasted to me and others that he has eaten human flesh. [ … H]e went on to say that eating human flesh is not uncommon in his home area. (Kyemba 109–10)

And, moreover, the article seems to state Amin's cannibalism as a fact and is actively engaging in trying to explain it. Based on that then there seems to be some veracity to this claim, but its important to remember that this professor may be mistaken. I unfortunately do not have the credentials to judge him as a scholar.

Here's the Source for further reading. Leopold, Mark "Sex, violence and history in the lives of Idi Amin: Postcolonial masculinity as masquerade.." Journal of Postcolonial Writing 45.3 (Sep2009): 321-330.

Edit: I meant say that none of its cited sources are immediately obvious to follow for further direction other than perhaps Kyemba's A State of Blood, but that is not a peer reviewed source and its hard to say what biases Kyemba brings to the table without extensive reading