If he has no hay for 3 weeks as you said you need to get hay asap. If you go to vet tomorrow please tell them that your rabbit had no hay at all before they prescribe any medicine. Because medicine won't fix your rabbit's diet, if his poos are small as you said (but not like in your first photo) he maybe has GI stasis already and will need medicine, but tell vets that his diet was only pellets for the past 3 weeks.
Based on photo you posted I would get hay asap and wood feed unlimited, his poo should improve after a few days.
You need to learn some basics about rabbits, in first link Mariam posted there's a very good simple site with lots of pictures all about keeping rabbits at home for new rabbit owners, check it out, if you follow recommendations your rabbit will be fine.
https://rabbitsindoors.weebly.com/
P.S. Since your rabbit is still only 2 months old you don't need to feed vegetables and greens, fruits and other treats, that all after he is 6 months. For now he should only get hay, water and good pellets.