i had a crainotomy 2yrs ago and i keep a temp of 99.8 all the time. i keep a low grade fever every day . my surgen said my hypotlamus was damanged by the pressure of my tumor. i cant stay in the heat it makes me sick and feel like im gona pass out. that is just my experance ,but before i never got above 98.1 unless i was sick. i would talk to his doctor and have him tested to see if he did have another infection.
I'm sorry, I don't have an answer for this. Your father's body is still healing from the stress of surgery if it was just 2 weeks ago, so perhaps this will correct itself with time, and good immune support.
Definitely stay on top of this, though. Let his doctors know every time his fever flares up.
Sorry I couldn't be more help.