I agree with B to add in cooling baths to lower the temp a bit, although as you say, a reasonable fever is a normal sign of the immune system kicking in.
And yes to fluids in any form without worrying about food. Dehydration is not a fever's friend, and water flushes out whatever's inside him that needs to be gone. Popsicles, jello, ice chips, Gatorade, watery fruit/veg like watermelon and cucumber, ice chips. Make a game of it if you can - every time you roll a 6 (since he's 6) playing a dice game, he chooses which of 3 cups of liquids to drink from. Every time he drinks 4 ounces, he gets a popsicle.Whatever works. He will complain about peeing. It's okay - reward him when he goes! You're not spoiling him - you're giving him incentives.
Try to focus less on "this isn't my kid" - I totally understand, but it's not helpful. And he will pick up on your stress. He will recover. He's talking, he's lucid, that's all good. Use this quiet time, abnormal as it is, to do some quiet activities - a craft, looking at baby photos, learning a simple board game or 3 that don't require too much concentration. It can be a very sweet time in some ways.