What about teaching him to cook? In my opinion, every 15 years old should be able to cook, wash dishes and help with the kitchen tidy up?
The easiest time to teach them is before they are 12 yo but it's never too late. You just get a lot more arguments once they hit the teenage years and you'll get to practice your patience
(Don't cave in)
Just say that you want him to be able to cook when he goes flatting. Show him how to cook his meat, eggs etc. Once he can do it, say that you don't like handling animal foods anymore and ask him to cook those foods himself.
I predict that he will be too lazy to cook that much and that will naturally cut down the amount of animal foods he'll eat. If not, at least he'll know how to cook and you have less work.
By starting this with saying you want him to learn to cook, you avoid the argument over meat eating versus vegan.