He has been eating them for 50 years and the Big Mac maybe for the first 20-25 years were probably not that bad for him with more natural ingredients, but as their food became more processed is when it became detrimental and maybe he has developed some detox process for the crap they put in it that keeps him somewhat healthy from eating every day.The entire thing is filled with chemicals not meant for consumption by either man or animal. Look it up.
But yeah I seen stories of McDonald’s hamburgers looking pretty much the same years after they were sold just dried up, no mold do decomposing.