Flemish Giants' are big rabbits, so they eat quite a bit. Since you're giving him unlimited greens, I would recommend 1/4 cup of pellets per 4 lbs. So if he was say 15 lbs (the average for a Flemish) would be roughly 1/2 and 1/4 cups of pellets. Obviously, if you want to make is diet to be a small amount of pellets and lots of greens/hay you could go 1/8 (2 tbsp) per 4 lbs. but I worry that may not be enough for such a big rabbit.
Of course, if you notice he is having diarrhea or extra cecotropes, lower his pellet amount or lower the intake of certain greens he gets (for example, kale and lettuce tend to cause problems, lettuce can mess with the intestinal tract, and kale is high in calcium and protein).
Let me know if you have any questions or concerns!
Are you feeding an alfalfa based higher protein pellet(what protein percentage)? Does he get unlimited hay and is he eating a pile of hay at least the size of his body, per day? What kind of hay(type of grass hay, alfalfa, or both) and what cut (is it soft, coarse, or mix of soft and hard stems)?