tomatoes stop floppy rabbit syndrome, but I don't think the majority of rabbits are in any danger of that - correct me if I'm wrong someone please!
You can introduce veg around now, but it's crucial to be patient and only introduce one new type of veg a month.
Start off by giving him just a coin sized piece of whatever veg you want to give him - then miss a day, so he has one day with veg and one day without - That will help his stomach build up levels of bacteria to digest it, and you can see if it upsets his tum.
So do this one day on, one day off with only one type of veg, over about 2 weeks, gradually making the pieces of veg bigger, assuming it doesn't upset his stomach - If his poos get loose then stop and leave it a while - If he has diarrea then take him to the vets immediately as baby rabbits get dehydrated easily.
Make sure the veg is washed clean too. I just found bird muck on the greens for my lot!
So after 3 weeks he should be eating one veg - Then you can go through the whole process patiently again with a different type of veg, still giving him the first veg daily, but the new veg in small bits every other day.
They don't want fruit, or sweetcorn, or iceberg lettuce, or tomatoes I'd say - and not a lot of carrot even when grown up - just a few inches a day. Greens are best, and mine love brocolli too but it can cause gas I've heard.