Sounds like it was way to strong for him. But this might help go to the store and get some Murphy's Oil Furniture soap. I hear about it for some Shar pi breeders(their dogs have a lot of skin problems) and tried it and it does work and helps the dogs skin. The breeder of my first 2 Terv's tried it and she was pleased with it.
It will keep them off for only a short time but it will not hurt him. Make sure to make the suds thick as this will kill the fleas
Shampoos of any kind are a relatively poor and ineffective way of getting rid of a pet's fleas anyway. They may be good for cleaning an animal, or for treating certain dermatological diseases, but not for doing anything significant in terms of getting rid of fleas. I hardly ever use flea shampoos anymore in my practice- too toxic for some, and too ineffective for all. The safe orals and topicals are far better way of getting rid of fleas as they have month long residual effects (baths have 0 residual effect). I consider flea shampoos as 'old' medicine.