He needs advice from a doctor or if he is worsening, straight to a hospital.
Does he have a sick day plan, which outlines what to do in these circumstances?
I cannot give you professional advice but my sick day plan is to keep replacing lost fluids and food with SWEET soft drink or a propriety product like Gastro-lyte, from the chemist.
Little sips often rather than big mouthfuls. Passing urine every one to two hours tells me am staying hydrated.
I go to hospital if I have not been able to pass urine for 6 to 8 hours ( dehydration) and it will be less than that with an 11 year old, hence the need for medical advice.
I might reduce for insulin 10% but the stress of the illness and the sweet drink usually stops me going hypo.
With persistent blood sugars over 15 or hypos, in either case where dose adjustment say 20 % does nothing, is a reason not only to be in touch with doctor/nurse but perhaps think about hospital.
Blood in the vomit or stool, appearance of a rash or fevers say greater then 38 to 39 degree C are reasons to get to hospital pronto.
Please check with your doctor as these guidelines for me may not apply to your 11 your old.
Best Wishes for a speedy recovery.