He might be fighting off a mild bug. I don't tend to take my kids' temperatures (except when they were very small and went to daycare) and I think we may have when they've been exceptionally ill. I wouldn't keep taking it though - I think you'll drive yourself nuts (just my opinion). If the doctor is saying he's fine, and he's not lethargic and playing fine, eating well and not dehydrated - I'd just let him be.
Unless he shows other signs - it's probably just something going around, and he's just fighting it off - like his body is supposed to. I doubt most people would notice it.
Just my thought. My kids have been a bit warm before, and it passed - and later I found out that something was going around the school.
ETA:
Just reading through the other moms' responses.
Has your son had issues in the past with his health - I looked back and saw that he may have (asthma?)
I don't know my kiddos' regularly temperatures. You're taking your son's all the time. I do think it's a valid point that it may stress your son out. He needs to have trust in doctors.
You may have some anxiety and it's ok to admit it. I have a friend who does about her daughter's health. She has anxiety in general so it's natural for her to have it about her child (loves more than anything). I have slight anxiety so I get it. It's not easy.
If you don't have confidence in your child's pediatrician, then it's time to find another one. Get recommendations. It your worries are getting the best of you, or you feel you need support - you can always talk to someone. I have at times. It's a great relief.
I'm not discounting your child may be fighting off a bug - kids do, and also his 'normal' may just have changed. Hormones, sleep, food, activity, ... can affect our bodies. But I imagine stress can as well, and so maybe take a break from this for a while and just go a week and see how things go :) Best to you