I would say just keep on feeding him what you already have. You may not want to introduce too much to him, that wouldn't have already been in the area he was at. So the grass, dandelions, and clover is perfect. You could even go back to where you found him, and see what kind of edible plants are in the area. Other plants that you aren't sure if he would have had before, or even veggies, you could introduce one at a time, in small amounts at first, and then gradually increase the amount as long as there aren't any mushy poops. You have to really feed a lot of greens to make up for not eating pellets or grass hay, but you also don't want to feed him so much that he doesn't get hungry enough to try the hay, because it's something he is going to need to start eating as a domestic rabbit. So you'll always want to keep a little bit of grass hay in there, and eventually he should get curious about it and start eating it. But if you plan on keeping him, you may also want to gradually get him used to things that you would normally feed a rabbit. Eventually nutrition wise, he will either need a limited amount of pellets each day, or a good variety of veggies and a lot of them, so that he is getting the proper nutrition, and unlimited grass hay. I agree to hold off on the pellets and only start introducing them to him once he's been with you a week or two and doesn't have digestive or poop problems, and is eating a bunch of hay. When you do start pellets, make sure to introduce them gradually and start with small amounts(just like with new veggies). Then gradually increase the amount each day and spread it out over 2-3 weeks, to give his digestive system time to get used to the new food. Make sure to keep an eye out for changes in his poop. I agree to hold off on anything sugary for now, as it can cause digestive problems. And if you introduce alfalfa, do so gradually, as it can cause digestive problems as well, if introduced too quickly.
Here are some veggie lists you can reference when you start him on them, just always keep an eye out for any digestive problems and soft poop. If you only introduce one thing at a time, if it causes digestive upset, then it makes it easy to know what is causing the problem, so you can stop feeding it for the time being. Also just be aware that cruciferous veggies can cause gas and digestive upset with some rabbits. If you haven't already, he'll also need a water dish.
http://www.3bunnies.org/feeding.htm#top
http://www.rabbit.org/care/veggies.html
Good luck
I can't wait to hear how he does in his new home. It sounds like he is already adjusting really well.