Welcome to BB!
Considering how young your bun is I would suggest he be on unlimited Young Rabbit Food and unlimited Alfalfa hay. This seems to be the best in helping our baby buns gain weight and grow up healthy and strong! He should be on unlimited of both until about 6-7 months of age when you begin reducing their pellets and introducing timothy hay into their diet. (You can also introduce other kinds such as Orchard Grass and Oat Hay but typically Timothy is very popular and you will eventually switch them off Alfalfa because it’s mainly to help them gain weight.) Have you began introducing veggies to him? If you have, or are deciding to, I would introduce no more than one veggie per week so you can watch his poos and make sure nothing is upsetting his little tummy.
Have you began considering options for neutering? This will help with any litter-training and behavioral issues you may have with him. If not already, then soon, he will definitely be getting those hormones kicking in and BOY will you know it!