My 5 yr son is to enter next week to tonsil n adenoid remove surgery is it recommendable since he's had fever at least once per month since december ?

No: What has been causing your child's fever? If he has had other symptoms besides those referable to the throat area (such as runny nose, cough, GI symptoms), then i would say for sure, "no." if it is only sore throats, then the answer would be "maybe.".

Probably fine: Were the tonsils the source of the fever? If so then yes it makes sense. Could he have pfapa (look up the "symptoms of pfapa")? Then tonsillectomy can be a cure. But i would not want to operate on tonsils during active infection (give 2 weeks after infection to decrease inflammation) as that can increase risk of postoperative bleeding.