Stay away from rabbit mixes that look like the bunny version of kids' cereal. Your bunny just needs the pellets, not fattening seeds or colorful crunchy bits found in some pellet blends.
Follow the guidelines on the bag of timothy pellets to tailor the serving size to your rabbit's weight, don't overfeed and scale back even further if your bunny only wants to eat pellets and not hay. Remember that alfalfa pellets are just for young rabbits or elderly rabbits who could benefit from the calories. Fill up your baby bunny's food cup if he begs for more. Gradually wean a bunny off unlimited pellets to adult rations near a year of age.
Ask your vet if you're not sure which pellets would be best for a young or underweight rabbit, and to rule out any potential health problems of a rabbit who is not eating.