He looks young to me too., so probably want to start him off on an alfalfa based pellet, preferably with no corn by products added and alfalfa as the first ingredient, as he needs the extra protein for growing. It's best to gradually introduce pellets over a week or two, starting with a small amount and increasing a little bit each day. As for total amount you want to end up feeding on a daily basis, for young rabbits I like to feed twice a day and feed just enough that they run out of pellets 3 hours before their next feeding, so in that time they will eat hay which they need the extra fiber for good gut function. If you do have a giant bun on your hands, they don't really finish growing until about a year old, so that would be about the time when you can transition to a grass based pellet if you want, and also start reducing daily amounts til you are giving the adult maintenance dose(1/4-1/2 cup per 5 lbs body weight). Though personally I've gotten fed up with certain issues with pellets and don't feed them at all anymore, and only feed hay and forage now for my adult buns.
Timothy hay is what a lot of rabbit owners feed just because it is readily available at most stores that sell rabbit supplies. But any grass hay is fine(orchard, meadow, bermuda, etc). It can be expensive to buy the pet hay, especially if he really is a flemish, you'll be feeding tons of it. The cheapest way to get hay is at your local farm store. Most of them will sell bales of timothy or some sort of grass hay. You want horse quality hay that is a mix of coarse and soft stems, not too dusty, little to no weeds, and has never been wet(most important part). Look for any black discoloring on the outside of the bale. This will indicate the bale has gotten wet at some point, and you do not want this. When opening up the bale, it should never be wet, musty, whitish or black, or smell sour. This all indicates mold, and can be dangerous to feed to rabbits if it is. It should be dry, a greenish color, and smell sweet, grassy, or sometimes a bit like tobacco. You do want to free feed the grass hay, so unlimited access to it all day, and refresh with new hay daily. You want to try and monitor the amount that is being eaten each day. It's best for a rabbit to eat at least a pile of hay the size of their body per day.
If you do introduce veggies, I would wait a few weeks til you've settled with the pellet introduction, then gradually start introducing veggies one at a time and small amounts at first. I personally prefer to stick with leafy greens and avoid cruciferous greens and veg, as many rabbits seem to get an upset stomach with them, which can be a very serious health risk with rabbits. I also like to feed forage, like apple and willow leaves, which my rabbits LOVE.
With all of the new food introduction, keep an eye out for mushy poop, or your bun not eating or pooping, as these indicate digestive upset. For mushy poop, if it's pellets, reduce the amount you are feeding and see if that clears it up. If it's veggies, stop the veggie. You may not be able to feed that veggie, but if you do want to try it again just introduce it more gradually with much smaller amounts. If the mushy poop returns it means that your bun can't tolerate that food. When introducing new foods, if it causes your bun to stop eating and pooping, stop the food, infant gas drops might help, but if after about 3 hours your bun isn't back to normal, it will require vet intervention, with pain and gut meds.
This has a good list of the various veggies and forage that is considered safe for rabbits.
http://www.medirabbit.com/EN/GI_diseases/Food/feeding_en.pdf
I know this is quite a lot. Feeding a rabbit is much more complicated than I ever imagined when starting out. If you aren't too overwhelmed with all the info, then you're ready to take on your new bun(if no one claims him missing). Good luck with it all and feel free to ask if you have any more questions
