Just wondered what medication OP's son was using before changing to 2 injections (presumably insulin) a day? If he's changed from tablets, then I wonder why he's been put on relatively inflexible bimodal insulin, instead of 1 or 2 x long acting insulin and 3 x short acting insulin with meals. While there may be no financial benefits available, anyone with a diagnosis of coeliac disease, whether adult or child, should be prescribed a certain amount of gluten free food per week, and as all children, and adults on mefication for diabetes (plus some other conditions) get free prescriptions in England, then they would not pay for this food, which costs more than equivalent food with gluten.
Definitely worth your son (por his wife) onsulting Citizens Advice Bureua or welfare rights, whatever they have in their area, to check what the situation is - DLA is being replaced by PIP for working age adults in some areas, but children can still claim DLA, and depending on other conditions of amount of care provided and wages earned, an adult might be able to claim Carer's Allowance.