I think you're right Lucy - it may well be due to high nitrites. And yes to bigger and more waterchanges. Nitrites at even 0.50 is bad and breathing would be difficult if the levels are at 2.
Unfortunately at this stage of cycling, it's often necessary to do 2 massive changes daily and while there's nitrites in your tank you could also add 1 teaspoon of sea, rock or aquarium salt per gallon as it helps protect the gills during this phase
PS. As he is a new fish, it could be parasites but try to get your water in better condition and see if this improves his condition otherwise you may need to look at treating with a parasite med