I had this with dc when they were 2 years old.
Not really any other symptoms except I knew something wasn't right, and I couldn't get their temp below 39-42°C for longer than 2- 3 hours (still high though), and I was regularly giving Calpol and Nurofen.
This went on for about 3 weeks altogether, with me getting increasingly worried, but being fobbed off.
Originally, after the first 2 days I went to my GP and because they'd had a cough (although it was mostly gone by then) about a week before this temp started, we were given a week's worth of antibiotics, "just in case".
They didn't work, their temp wouldn't stay down, but GP thought I was an over-anxious, paranoid mum and kept saying kids get colds, stop worrying!
I kept ringing every few days asking for advice &/or appointments, which just confirmed to them that I was paranoid, and in the end they agreed to give me a letter to go to A&E so "they could tell me he was fine and I was paranoid".
Turned out they were wrong, and dc had pneumonia. Spent about 8 days in hospital on iv antibiotics, etc.
Now I trust my instincts, but at the time it was my 1st dc, and it was drilled into me not to bother A&E for trivial things, and the GP kept saying a high temp didn't mean anything bad on its own.
Hope your lo is ok, OP. 