Timothy pellets at 6 months, especially because you’re only feeding 1/4 cup a day. This will really help to ensure he’s growing. When you transition to adult pellets, make sure you do so gradually by mixing it in with his current pellets that way his body is able to adjust to the new food.
For veggies, start with leafy greens. Romaine is the safest and so are red/green leaf lettuce. Introduce new veggies one at a time and slowly over a few days so you can monitor his poops. When it seems like he has acclimated, you can add another new leafy veg into his diet. Try to steer away from cabbage or things in that family as they are known to cause gas in some bunnies. Herbs are also a good choice, parsley is known to have a diuretic effect, so keep that in mind if you try that and he seems to pee a lot. Introduce fruits and sugary veg like pepper and carrot at 6 months in much the same way.