it's a really hard decision isn't it and one you could do with a crytsal ball to make. My eldest had real problems with her breathing less that 24 hours after her first SF symptoms that really scared me and for that reason am glad had already (rather reluctantly) started her on the tamiflu, she had a wheeze from early on though that the doc was particularly concerned about - an indication that it was straight away affecting her breathing. She was unlucky though most kids don't have such problems. She's fine now too, she had to have a course of steroids and an inhaler for a week but she's fine.
My youngest had taiflu cos I was frightened that she might get as poorly as my eldest but she never did.
I had it too but think I took tamiflu too late which was pointless in a way but I thought I might just make the 48 hour window and had crappy advice from NHSdirect that delayed me getting it, as my daughter had such a bad reaction to the SF I should've been given it you see.
I had terrible flu and a secondary chest infection, am still fighting off the flu 10 days later and have got rib pain and even still got a temperature. I wouldn't mess with the SF it's as strong as an ox and in my opinion you have to be as strong as an ox to battle with it, I wasn't I'd had a horrid time worrying about my girls, hardly had any sleep in days and spent a night in A&E with my eldest so didn't stand a chance against it.
You'll have to decide what you thinks best the tamiflu works best the earlier you take it and also helps protect others in the house from catching SF as it works by limiting reproduction of the virus.
It's a gamble either way you could give it him and then he could be fine tom and you might wish you hadn't or if you're unlucky like me with my eldest you might be really glad you gave him it as a precaution - sorry isn't an easy answer.
Has he a sudden cough/sore throat/headache or any other symptoms except temp and aches? Aches are more common of a flu I believe but I'm not a medic, I certainly had terrible aches. Hope he has a good night and is better soon, even if it is SF am sure he will bounce back in a few days like my girls did x