If he’s not used to fresh veggies you can start with just a few leaves of romaine lettuce. Other greens they often like and that are good for them are dandelion greens, fresh herbs (cilantro, mint, parsley, basil, ruccola f ex). Practically all leafy greens are good, but the important thing is to start with a little and gradually up the portions to like 1 1/2 cup daily (or more). If you have access to apple tree twigs and/or leaves and/or willow a bunny will love those.
1/8 cup of Oxbow pellets is the recommended daily amount of pellets but adjustments have to be made according to the bunny’s size (weight). 1/8 cup would be good for a bunny of 4-5 pounds, but it also depends on activity level of course. Any change in diet should be done gradually, so if you switch from one brand of pellets to another, make the switch gradual by mixing in the new pellets with the old brand.
Good luck with your new bunny!!!