3 weeks old rescued kit

I had 2 lionhead bunnies and recently I've found a new litter in their cage. There were 6 new kits, one unfortunately died. I separated papa rabbit and the babies were growing healthily until they've reached 2 weeks old. Their eyes had already opened, they had become little balls of fur, and had already started eating. Unfortunately 4 days ago, a dog managed to sneak in on my terrace and killed mama rabbit and 4 of the kits. Only one survived.

My issue is that baby rabbit is not pooping at all. He is 3 weeks old and is quite healthy. He runs around in his play area, sleeps well, eat his cabbage and carrots, drinks water, loves cuddles, pees but no pooping! I am very worried about this. Is there something I could do ?

I'm so sorry about your bunny loss.

Thank you . I was devastated in the beginning but now my main preoccupation is to make sure baby rabbit survives.

I have to add that I did consult a vet about this problem. He assured me that baby rabbit was perfectly healthy and this was no big deal. He told us to wait, he'll poop when he feels the need to but it's been 4 days since.

Also I rarely leave the baby alone. He's well surrounded and even has this little plush animal that he loves to sleep with.

Your vet knows nothing about rabbits, then. A rabbit not pooping is not perfectly healthy.
Baby bunnies (until about 3 weeks) need to be stimulated to defecate and urinate, by imitating the mother licking their bums with a cloth wet with warm water.
what is he eating?
This article might help:
http://rabbit.org/caring-for-orphans/

He started pooping yesterday but this morning I noticed his anus was red. He eats a lot of green leaves and some pellets. Thank you for the link.

How's he doing??

He's fine. He's eating and pooping well too. We made a playground for him out of cardboard with a little hide in it and he loves his new place. He's always playing and jumping around whenever he isn't sleeping or eating.