Menthol and health, is there a doctor in the house?

Menthol causes vasodilation, which can be seen it in the resulting redness when applied to skin. It's typically minor, but the redness is from blood pooling in dilated vessels. The corresponding effect is a dip in BP. If his BP was already low or he was dehydrated (or both), it's possible that the amount of blood pooling (both internally and externally) dropped him low enough, fast enough that he felt faint.

FWIW, I'm not a doctor. I have fainted on occasion, though.

Exactly this.

And any kind of menthol-containing essential oils or ones with 1,8 cineole , including eucalyptus, spearmint, peppermint, shouldn't be used around children under 6 and sensitive individuals. Not all kids will react, but some will have that exact reaction. I stopped keeping Vicks VapoRub in the house after learning that.

I've been trying to check multiple trusted sources before even buying a new essential oil anymore. Not worth the risk to us with littles running around, even if they're safely out of reach. Though I definitely have the mints, etc, on hand, I just use them in ventilated space.