If that is all he is eating then I’m sorry to say that his diet is not appropriate and needs to be rectified to include plenty of fresh leafy veg (needs to be introduced slowly). Inappropriate diet can also be the cause of bladder issues.
Hay and fresh grass needs to be their main food intake. Rabbits also need at least one cup of veg (depending on their size they can need two cups of veg per day), grass and any fresh forage you can get hold of each day.
They should only have one egg cup full of nuggets per day.
I’ve no idea what a carrot biscuit is but if it is a commercial treat then he should not be eating them. Carrot and carrot flavourings can also cause red urine.