If moist pellets keep him eating for himself, it will keep him going and is less hassle than syringe feeding. You could try cutting up a daily handful of fresh grass into very short pieces (eg 5mm with scissors) and mixing it in - that will increase the fibre naturally and he may manage to eat it if short enough as it won't need chewing the same as long strands. You could also try the leaves of dandelion, bramble (both available now) or willow (which will soon be out). My dental bunnies found it easier to eat leaves off twigs hung just above nose height, rather than trying to pick them up from the floor. It also gave them something different to do.