From your description, the cause of his bald patch is most likely due to alopecia areata.
I agree with Dr Joshua Chong who has explained in detail the autoimmune cause of this condition, and how it can be triggered by stress sometimes.
We do treat alopecia areata regularly in our Dermatology clinic and good improvement can usually be seen with monthly steroid injections into the bald area after 2-4 treatments.
Occasionally, we also prescribe topical treatments to apply to the area in between injections. If he only has one bald patch, the likelihood of good improvement is very high. There remains a chance that the condition may recur in the future.
Warmest regards
Dr Stephanie Ho, Dermatologist
