Prohibiting what is lawful

Salam alaikum wa rahmatullah wa barakatuhu
. Is it okay for any muslim to prohibit himself from eating what is lawful in Islam ?
A brother has decided not to eat any animal product like meat, fish, chicken, eggs, milk, youghurt, butter anything whose source is animal. Though he has been eating since birth being born in a muslim family he about 40yrs of age, delivers khutbas in mosques and is a well informed person. Apparently he has no medical reasons either.
Awaiting your response
. Jazakallah khair