Rabbit doesn’t eat much

by Pearl
(Boston, MA)

Hi,
I have a single rabbit, which I adopted from a another family.
He is 6 month old.
He likes to explore a lot and run when I let him out of his pen. He seems to sleep or lie around in the day and appears to be most active after 8 pm.
But what I noticed is that he doesn’t eat much hay or pellets. He does not seem to poop during the day.

However when I put him back in his pen at night and see him the next day, there will be a decent amount of poop In his litter box.
He is not neutered yet.

What can I do to make him eat more? I am afraid of gut stasis that I have been reading up on.

Also I was told this is a dwarf, can someone confirm this?

Thank you,
First time pet owner