Hello, I'm Dr. Zoe. First, I do want to ensure the impaction issue is resolved. This is a serious long-term issue in older male guinea pigs. If not, I can give you some discussion on this problem. There really is no cure.
Second, with the bald spot and dander, if he is also itchy then I suspect he may have mites. This is the number one problem associated with hair loss, dander and itching. Your vet can prescribe a product called Revolution for puppies/kittens which is a topical you apply once every 14 days for 3 doses and will clear. I do not think the shampoo caused the hair loss.