Hives can happen with no apparent trigger. In my life, I had three separate outbreaks as a kid with no apparent cause.

Identifiable causes include:
Infection
Medication reaction
Sting/bite
Latex
Food

It’s ok if none of those fit. Again, half the time there is no known cause.

Primary treatment as long as no breathing trouble, vomiting, fussiness beyond itching or other weird symptoms is antihistamines.

Give him either children’s Zyrtec, Allegra or Claritin. While they say once a day dosing, it’s generally ok to do twice a day. Stay on it scheduled. Might take a week or two to fully go away. Rarely he might need oral steroids.

If you are worried about anything or he seems like he’s not ok, see a doctor.

This is online advice without seeing or examining your kid beyond a photo so follow it at your peril. See a doctor, not a CB doctor. But this is common treatment for this condition.

Good luck!