My father's leg became very red hot and swollen from the knee down and was hurting, dr. said he had been bitten by something, what could cause this?

Did he develop: an infection from the bite with cellulitis?

Cellulitis: If leg is red warm and swollen, this could be sign of bacterial infection called Cellulitis. Rarely, this can be serious (flesh eating bacteria). Antibiotic is warranted if cellulitis. Need to consider Deep Vein Thrombosis especially if leg edema and calf tenderness

Can be from: an insect sting but also cellulitis or an allergic reaction, Call the doctor if you have questions or if getting worse have him seen by the doctor asap.