Hello, in a surgery to remove tonsils, is there any risk such as brain damage during general anaesthesia if my 5 yr boy has had febrile seizures?

Tonsillectomy. By the time most children reach five years of age they have outgrown febrile seizures and should not be aproblem for your child to have surgery for his tonsils.
Risks. Discuss with the anesthesiologist. This should not be an issue.
No: Febrile seizures only happen with fever, and no one is going to do elective surgery while fever is present.
Unlikely. I am not an anesthesiologist but a problem with seizure activity is very unlikely. Remember that general anesthesia represents a very high level of sedation. This reduces brain activity and in general makes seizure less likely to occur. Febrile seizures are different from other forms of seizure and in this instance in particular there should not be an increase in risk.
No additional risk. A history of febrile seizures does not increase anesthesia risk.
T&A. General anesthesia is about the best anti seizure medication available. If the anesthesia is handled appropriately, there should be no seizure activity.
Minimal. While there is always a risk from anesthesia that risk is very small. Febrile seizures should not increase that risk. Please ask your anesthesiologist about this topic to allay your concerns.
Surgery and seizures. You can get fevers of up to 101-102 after (not during) any surgery for a few days. Any higher than that and you need to have your child assessed as there could be another cause such as pneumonia or flu. It's crucial to keep predisposed children hydrated after tonsillectomy. Treat fevers with medicines your ENT says to take. Surgery & anesthesia are usually safe with appropriate precautions.