Yes, a combination of things.
He’s in a new place, any bunny of any age will poop to mark new territory, and also in a new place they can just be unsettled. He will get used to things and hopefully poop in the box in a while!
Also, he’s a baby and at a very young age they don’t yet have the concept of using one spot to go, however one of mine who I got at a young age only took a week to settle in and begin using only the box at 8 weeks.
Neutering is also something that will solve that need to mark territory, and it is just recommended to neuter (at about 6 months, but it varies) because it is healthier for both the bunny and nicer for you.
Basically, you have a baby in a new environment and it will take a while to settle. Just keep putting the poops in the box, and put the box in an area where he seems to go the most. Also only put litter/bedding in the box and put hay in there too so it’s clear where he should go. I’m sure he’ll grasp it soon once he settles.