Dogs can suffer from seizures, and the most common cause is epilepsy; however, a seizure is a neurological response to a disturbance in the electrical center of the brain, and there can be a myriad of causes. The vet will try to determine the cause by extensive testing, but if no cause is found, the seizures are labeled 'idiopathic' (no known cause) and anticonvulsive medication can be prescribed to see if it will help. For more information on seizures in dogs, please see:
I find no research which demonstrates that Pit Bulls are necessarily prone to seizures. However, because you mentioned that you suspected the dog shampoo your son used, if it was a flea/tick shampoo, this very well could have been the cause of the seizure. Many over the counter (not bought from the vet) flea and tick remedies/shampoos, etc., contain insecticides which are very toxic and dangerous to dogs and have been known to cause adverse side effects including seizures.
It's also possible that the dog got into a toxic substance like a cleanser, antifreeze, poisonous plant/flowers, etc., and this is what caused the seizure. You mentioned blood tests were done, so I assume the blood glucose readings were within normal range; I mention this because diabetes can cause seizures, also.
If you don't feel satisfied with the vet's treatment, tests, and diagnosis (or lack thereof) of your son's dog, by all means, consult another vet for a second opinion. More testing may be necessary if the seizures continue, to get to the root of the problem.
I hope your son's dog will be fine after he's diagnosed and treated appropriately for this problem.