Hi BunnyAuntie and welcome to RabbitTalk. :hi:
I cannot stress too strongly the need to
go slowly with any new foods except grass-hay. A rabbit that was raised on junk pellets needs plenty of times to adjust to new foods, especially greens. They simply do not have the gut flora necessary to tolerate the change and can become very sick and even die.
Continue to offer the hay. It will help with the transition and is excellent for the digestive system, but a rabbit like Roo may not like it at first. Consider him like a child raised on fast food who sits down to a wholesome home-cooked meal. "You can't be serious" about sums up the reaction.
Rabbits generally like greens, and when raised on them from the beginning, they can digest them easily, but rabbits introduced to them later need to go slowly. One blade of dandelion or plantain (the lawn weed, not the banana-thing) the first day and add a leaf every day or two until you're feeding a "normal" amount. The best greens are the common weeds listed in the Safe Plants sticky.
safe-plants-for-rabbits-list-t55.html
Never feed iceberg lettuce and don't feed vegetables from the cabbage family until the rabbit is well accustomed to other greens. Always go slowly with any new food.
Change is always stressful to rabbits, and sometimes upsets their digestion. Water is vital, of course, but rabbits often do not eat much those first couple of days in a new home, especially if there is a lengthy journey to get there. Continue to offer the preferred pellets for the time being, but look about for a good quality pellet. Others can advise you better than I about pellets; for most of the time I had rabbits, I fed them a natural diet.
When Roo is settled in, eating normally of the junk pellets and nibbling hay and bits of greens, you can start to make the transition to the better pellets. Begin by adding just a tablespoon of the new pellets to the old ones. Limit quantity to what the rabbit will clean in a day. You don't want the rabbit to go hungry, but you want to encourage him to try the new pellets so a slight decrease in the amount of the old feed will help. Over time you can increase the new pellets and phase out the old.
I hope this is helpful. Continue to monitor the bunny's food consumption and its droppings, it's general demeanour and level of activity. We're here to help explain anything that puzzles or worries you.