Hello and hope you are doing well.
Your brothers symptoms could be due to Balanitis, which is inflammation of the glans penis. It could be an allergic reaction to the shampoo. Also, balanitis usually occurs in uncircumcised men, in whom the smegma collection along with discharge is likely to result in inflammation. This causes pain, discharge, itching, difficulty urinating and sometimes impotence. This is very common in diabetics. Other causes are due to poor hygiene, obesity, certain medications like tetracycline and due to superadded infection with virus and bacteria. If symptoms persist, he may need to consult his primary care physician to rule out other causes.
Hope this helped and do keep us posted.
I think it just needs a little time to heal on its own. Keep it clean and dry by blotting gently after shower. It may take a week. Probably just a little dermatitis from the shampoo being a little too strong for the delicate skin.