Did you read something or did someone advise your son against eating beef? I'm not a doctor or a dietician who would be your best source of information, but everything I've read over the years focuses on eating a healthy diet while avoiding foods that do not agree with you. So, yes, if I want a good hamburger, I eat one. I lost 21" of my intestines (small and large) so no spicy nor creamy foods. I attended a Zoom conference about neuroendocrine tumors and the general consensus was you can eat whatever you want, just try to make it healthy. I admit, that's something I do need to work on.
One last thought - protein is the word. We forget there are other forms of protein other than beef so maybe that would be a good place to start some research. (Salmon, cheese, etc). I'm sure his doctor would make a referral to a dietician if you all would like to do that.
Good luck!