I had this as a child and outgrew it - very delirious and high temperature even with minor illnesses, from around the age of eight. Usual delusion (though I had several) was the furniture coming to life and being very threatening/angry/wanting to kill the whole family. The other main one was being in a white room with no way out though the walls and floor were slowly contracting and I had to calculate how long I had before it squashed me. I found it very frightening, but it suddenly became less severe when I was 15 or so. I think I was 16 when I had my last episode and it's never come back (I'm 28 now).
My last episode was mild - woke up in the night to find stripes and angry triangles floating in the air and the wardrobe and cabinet looking at me in a threatening way. My Mum didn't even wake up when I shouted, my idiot dad came through to me and didn't realise what was wrong, though I was ill and he'd seen it lots of times, and told me I'd had a bad dream . I knew when I woke up clear-headed the next day what had been wrong with me. It's never come back and the furniture has remained stubbornly inanimate even during illness/post surgery after a 7 hour GA as an adult.
The doctors were never concerned in my case. If you read old-fashioned children's books children often get ill and become delirious -in Elinor M. Brent-Dyer's Chalet School series, it's the sensitive, imaginative types that do
In a way, I'm glad it happened to me - it makes you see the world a bit differently, and you always remember that - a bit like a painting by Hieronymus Bosch (sounds a bit pretentious, but it's true )
If you're worried, can you speak to the doctor again? It must be scary to watch. I expect you already know to try and keep the temp down with cold flannels, and get help urgently if he has a seizure. I don't know that there's much else you can do - it is scary because he really believes it's real when it's happening and nothing will convince him otherwise. But if he's like me, he will grow out of it, and might even end up being glad he's experienced it.
Hope that helps - sorry I don't have a magic cure for you