Hi ! Good evening. I am Dr Shareef answering your query.
Most likely the lumps you have described are enlarged cervical
lymph nodes. Of course in young children normal lymph nodes could be palpable. But these are usually mobile,firm, and not hard and fixed. Any inflammation due to the insect bite could be responsible in the enlargement of regional draining lymph nodes and again they will be mobile, and tender to touch. His tummy problem might be an independent entity and might not be related to these.
If I were your doctor, I would go for a thorough clinical examination of your child with special reference to the head and neck region including mouth, ear, and nose. If nothing is conclusive, I might consider a short course of emperical broad spectrum antibiotic for him. If no relief, and the swelling persists or increases, then I would not hesitate to advise for a FNAC (fine needle aspiration cytology) of the lumps to arrive at a tentative diagnosis for further management.
I hope this information would help you in discussing with your family physician/treating doctor in further management of child's problem. Please do not hesitate to ask in case of any further doubts.
Thanks for choosing health care magic to clear doubts on your health problems. Wishing you an early recovery. Dr Shareef.