Yeah, it could be the pellets! 1/2 cup is quite a lot for a holland lop.
You’re right in that they should get more pellets as they are growing, but I think it would be OK to reduce the amount by a little, especially since he’s getting alfalfa hay.
You can start by reducing from 1/2 cup to the amount that he typically eats in a day. So maybe just remove a couple tablespoons a day until you figure out the amount. Or you can measure the amount left over after a day.
Of course it also is always good to check with your vet before making any major diet changes, but this advice has been given here before so I think it would be safe.
Regarding poops, someone shared this guide here and it is really useful! https://imgur.com/a/5N4lD
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.