Wire bottom cages are harmful for rabbit feet.
A very young bun, like your bun, doesnt yet have a mature gastro intestinal microbiome. A diverse and sturdy GI microbiome needs time to establish itself. The best food for a rabbit’s gut microbial balance is grass hay.
As LBJ said, you could try and cut out the alfalfa hay, since he has junior pellets. Alfalfa and junior pellets are both rich in protein. Rich food often causes poopy butt.
In order to get your bun to eat more grass hay, you can serve it in smaller piles and replenish several time per day. Newly served hay is yummier than “old” hay, even if its the same hay. You can also serve hay in several places in his area -“found” hay is yummier than given hay. Rabbits also tend to eat more hay if its served on the floor and in/very near the litterbox. Theae ways of serving hay mimick the way wild rabbits forage for their food. They like to poop when they eat. They often forage over larger areas, they eat a little bit here and a little bit there.
You can give a rabbit appropriate probiotic such as Benebac to help strengthen the gut microbiome.
Poopy but in a young bun is not a serious health problem. Here is a good article from the HRS on intermittent soft cecotropes. The article also has info on true diarrhoea, which is NOT what your bun has now. It can still be useful to read through that segment, because its good to know the difference between true diarrhoea and the relatively harmless condition known as intermittent soft cecotropes, ISC:
https://rabbit.org/intermittent-soft-cecotropes-in-rabbits/