Is he currently eating grass hay?

This is what Dr. Susan Brown DVM with the House Rabbit Society says in her article:

IMPORTANT: Before introducing any fresh foods to a rabbit it is best if he has been eating grass hay for a minimum of 2 weeks. The grass hay will help to get his GI tract motility and flora in good working order so that he will be able to accept new foods more easily. When introducing new fresh foods to any rabbit’s diet it is best to go slowly to allow the gastrointestinal tract and all its important microorganisms to adjust. Introduce one new food every three days and keep a watch on the stools. It is rare for a rabbit that has been on a hay diet first, to have any problems using this method, but if you note softer stools that persist over a couple of days then you might want to remove that food from your bunny’s diet. Keep a list as you go of the foods that your rabbit has successfully eaten you will then have a handy shopping list when you go to the store!

 From this article:

http://www.rabbit.org/care/veggies.html

I must have the only bunny gourmands on the planet. Pepper and Piglet came to me as rescues and at the time I didn’t have a clue and didn’t realise until later that they could have been more than a few weeks old. They started on hay but as they had been living rough in a free range situation where the rabbits were tossed any old veg (stale and shrivelled) and left to survive on anything else they could find I figured I couldn’t go wrong with most things. They were having sweet basil and parsley as a treat from the beginning as well as unlimited hay and pellets. When the kits came along they had two meals a day from Pepper plus hay and pellets on demand and by the 3rd week were stealing the basil, broccoli and Parsley out from under Pepper and Piglets noses. No buns ever had a bad tummy. Until Nermal came back to live then we had a few minor tummy upsets as I transitioned her back to the warren diet, I suspect she was not getting nearly enough good hay, just cheap bags of sweepings and they had been mixing her pellets with a Woolworths grain and pellet mix, apparently whilst they had her she only had a little parsley and carrot not the mixed herb salads she had grown up on. Maybe all buns are different to begin with or maybe its the diet they receive in the shelters and stores. But for me it’s lesson learned, if I have had the bunny from a kit a good varied but balanced diet is the way to go, if they come from somewhere else proceed slowly and with great care as even if they tell you what the previous diet was its not necessarily true or accurate information and its better to start off as if they had only ever had hay and water. All my bunnies now have a good diet of hay (unlimited) plain pellets (no frills, like grain and fruit mixed in) and a mixed salad twice daily consisting of Parsley, bok choy, basil, fennel, coriander, broccoli leaves, kale, nasturtium leaves and flowers, fresh grasses from the garden, celery tops (and limited) carrots, apple, craisins and strawberries. Plus for variety the odd card board box, furniture leg, willow basket and pet bed (nothing like a little forbidden fibre).

Make sure to only do ONE type of veggie per week. Too much variety at once could make his tummy upset again.

Roberta makes an important point. In homes/rescues where the mother gets daily greens, the babies will start nibbling at them at a very young age and this is one way to circumvent the “12 week” rule. But in cases where the baby hasn’t had any greens, waiting a bit is a good idea, and then introducing one at a time. I’d start with some basic romaine. Was the bunny vet-checked by the rescue, or have you taken him to your vet yet?

Making that list and his response to each green is a great idea. I also second Beka – please take him to a vet for a baseline exam and to establish your relationship with one you like so you can be sure it’s only a reaction to the greens and that he is in good health overall. If it’s unrelated, you want to catch that.
Romaine, green or red leaf seem to be the mildest lettuces for a staple food. I’d try those first, one at a time a week apart (unless he shows he loves one and his poos are normal, in which case keep him on it for a couple of weeks without change.) Kale, broccoli flourettes, and a few others are from the cabbage family and can give sensitive rabbits gas, so I would not try those for quite a while until he is normal in his digestion. Sprigs of mint, dill or parsley are gentle as treats eventually, too. Go slowly.