I wouldn't worry, as long as he is drinking, doesn't look to be in pain and that the pee looks normal. Rabbits are less active during daytime (becomes especially true when they grow up) so it's normal they use their litterbox less - at 4 months, Tybalt was freaking sleeping all the time with short bouts of running around during the day and he was unbearably active at night (he was sleeping in my bedroom at the time). Now, my 2 rabbits aren't much outside their cardboard house between noon and around 7 pm (especially now that it's getting colder) except when they find something naughty to do... I think I hear a hay bag being ransacked in the next room right now ^^'