Sounds like you already have a pretty good grasp on what to be feeding. Wild rabbits graze on grass and leafy weeds, like dandelion leaves. Make sure he or she gets plenty of water. Absolutely no dairy!
You probably already know that-but thought I would add it, just in case someone else who reads this finds themselves in the same boat someday. People think baby animals need milk-but dairy is really BAD for wild babies! And, ss you probably know, a six week old rabbit would be on its own in the wild anyway-so long since weaned anyway.
I recently started volunteering for a wildlife rehabber and you would be surprised at the number of animals that come in near death, because well meaning people find them orphaned or injured, and decide to keep them as pets. When people have no clue how to care for them, the animals health will quickly deteriorate, sometimes in a matter of hours-- especially when injured.
Rabbits generally do not like to be picked up. Their backs break quite easily, so extra care is required. I used to wrap my domestic rabbit in a towel to pick him up.
We just got two bunnies in yesterday-about the same age. It is understandable why people want to keep them. Baby rabbits are the epitome of CUTE!! However, it is rarely in the best interest of the animal.
Sorry if I am telling you things you already know!
I think you are right to get bunny to the rehabber as soon as possible! Thanks for helping him/her and good luck!