Thank you for your query at DoctorSpring.com.
I understand your concern and will do my best to help you.
Regaine Extra Strength for Men contains Minoxidil which is a vasodilator (substances which increases blood flow). Topical application of this over an area increases blood supply to that area. When applied over the scalp, it increases the blood circulation of the scalp and stimulates resting hair follicles and thus promotes hair growth.
The effects of topical medications is usually confined to the area where they are administered. Same with this. The discomfort in his testicles or the headaches do not seem to be related to its use.
Minoxidil is quite safe and can be used as per his doctor's advice.
He can take any OTC analgesic like Ibuprofen 400 mg for the headaches.
Testicular pain, more than discomfort needs evaluation. He can consult his doctor in person for the same.
I hope this was helpful. Let me know if you have any queries.