I don't think a vegan diet in prison will have good long-term results. Its doubtful he would get adequate protein. For vegans to get enough protein they need to eat a larger quantity of food than a diet with meat, eggs, and dairy (as those provide a higher level of protein in a smaller portion). I doubt he will be getting huge tempeh stir fries each night.
Using this calculator, putting in his age, gender, height, weight, and activity level (I chose "low active" - figuring he can do pushups in his cell), and it says he needs 65 grams of protein per day.
www.nal.usda.gov