I am sorry your boy is ill.
He can get a bacterial infection from drinking out of streams:
Seizures in a dog his age would most likely be caused by ingesting a non food or toxic item or a metabolic issue like diabetes, hyperthyroid, liver etc.
Not all seizures look alike:
Dogs can have strokes too:
He may just have become deydrated or hypoglycemic.
There is also a less serious condition that mimics stroke/seizure called Vestibular syndrome:
Please let me know how he is doing. I will be here late.
I hope he will be fine.
Sincerest best wishes,