I believe for a SD, you can't train them to alert you of impending seizures, it is as the previous poster stated: they have it or they don't. You could, though, have a SD trained to respond when a seizure is happening/over. They can be trained to alert you, or another adult, to stay with the person having the seizure, call 911, etc. I do agree with possibly hooking up with a good SD org. and seeing if they could fit you with a dog that has this gift.
Also, I don't know if this helps but my non-sd pups have, over time, picked up on when im going to have a seizure, when I don't feel good in general, or when I am just overall weak... much like the humans in the house have.
Best of luck on your search for a SD and with your son and his drs. Hope all goes well.