Hello. I'm Dr. Berry. Thanks for your question. If the testicles have not dropped by two years of age there may be a problem. Most will drop their testicles before 2 years of age. However there are a few that will take longer than 2 years. Just keep monitoring them every few weeks to see if the other one drops.
Should I wait to geld him until the other testicle drops?
Yes. If you leave the other testicle in without removing it the horse will act like it is not gelded. In addition the retained testicle has a higher rate of becoming cancerous if not removed. Sometimes the testicle can be felt in the inguinal ring. If not abdominal surgery or laparoscopic surgery will be needed to remove it.