I agree with GoodiesGirl's post. No-one can tell what is the roght or worng thing to do in this situation. It is a personal thing and I am sure you will make the right decision.
If it helps, the seizures do not hurt him at all. He feels no pain and would not even understand that he has suffered one. The seizures will put pressure on his heart though, ensure he stays cool when they happen as his body temp will elevate also. Perhaps offer him a little ice-cream after it to help restore his sugar levels, to cool him and as a treat.
Seizures in geriatric dogs are usually caused by dieseases such as cancer.