My son woke up with hives all over his body

Hives: Make sure first that he is breathing well and no tingling or numbness in his throat that you need to take him to the the ER. Give him bendryl according to his body weight. Then let him change him clothes and take a shower to make sure about any reaction he could have from his clothes. You might want start checking what was he doing before sleeping to identify the cause of the hives.

Benadryl (diphenhydramine): Give him benadryl (diphenhydramine) now and then every 4-6 hours for 1-2 days