M.G.
I'm not sure how to answer as not sure what you are asking either :)
Do you tend to get upset about your son's health? I noticed a lot of health related questions. No judgement - I get your concern as they are our little ones, but you've called the doctor, their advice worked, and you're going in tomorrow. Breathe :) I'm sure he will be fine.
Your child may not have been able to tell you his head hurt before with a fever because he didn't have the words to tell you - but that's not that uncommon. Mine get headaches with bad fevers. Some of it's from being dehydrated (good you're doing the water) but it could be he's got an ear infection coming, etc. Sounds like he's fighting something off.
Or he may have just banged his head on another kid's head yesterday and you may have missed it. Either way - if it were serious, there's be blurred vision, nausea, all kinds of stuff that you're not mentioning. Certainly not eating or playing.
When my kids get fevers - I automatically give them ibuprofen - and that stops the headaches from happening in the first place. Just check with the doctor tomorrow about that - but I know if I don't myself take meds when I get a fever, I get a headache. May as well be proactive - if they agree.
Just a side note - I would just the supplements you give him past your pediatrician if you haven't already. Mine is not a fan of added supplements unless there is a clear need for them (usually only after a problem with blood work). Supplements are most often not FDA approved. I just know ours has me bring in the bottles so they can check them out.
Relax and just take it easy along with your son :) Hope he's better soon
ETA: Thanks Diane for clarifying :)