My younger son had that once when I switched laundry detergents. I washed his clothes for a week in the old stuff and it went away. Then I just washed his clothes in the new stuff along with all the rest of the family's clothes and he got it back again, but it went away in a couple of weeks.
If you've tried any new topical products on him (soap, baby shampoo, anything) or on your clothes -- or perhaps you've gotten a new perfume for yourself, and hugged him or something, that might be able to do it.
I'd check the artificial ingredients in orange Hi-C. Both my older son and I are allergic to red dye #40 (it makes our noses itch if we get it on them), and sometimes it makes my stomach hurt; but it might cause a topical rash in other people. Think of it this way -- some people are allergic to peanuts and nearly die, while others may get a rash, and most people don't have a reaction at all. The same thing can cause different types of allergic reactions in different people.
It might be a heat rash too, and you can sprinkle corn starch on the affected areas to help keep his skin cool and comfortable and non-sweaty.