really sorry to hear about this. hope you and the pup are holding up okay.
cause of the seizures would determine a course of action but without history on the unknown pup's parents, there are like a thousand possibilities.
seizures in the first couple months are usually caused by congenital defect or neurological disorder. highly unlikely that treatment is possible in either scenario.
the pup may have been exposed to environmental toxins (like pesticides or strong household cleaners). if you think that's possible, you should do some spring cleaning and throw out anything that falls in that category.
there are a few nutrient deficiencies that may cause seizures. but if he's been on a decent food for the last month, it's unlikely. so prescribing a diet probably wouldn't help.
and there's always the chance of head trauma. if the pup was injured and has brain swelling or cerebral hemorrhage.
... like i said. alot of possibilities.
my best guess based on age and the fact he was abandoned on the street, a congenital condition resulting from poor fetal development (maybe mom was malnourished during pregnancy), incomplete formation organs and neural connections, maybe even respiratory defect that lowers blood oxygen supply to brain.
i would take honest look at his quality of life. if you think he's getting better and will make progress, you can stick it out (and bless your heart if you do). if his pain and discomfort causes him/you to live in fear of the next episode, you might consider euthanasia. shortening life can be considered a way of improving the quality of that life by taking pain away. physical and emotional. either way, you deserve a high five.
that's a tough situation. good luck.
~ jo and Gotti Boi (aka Thunder Thighs, aka Jughead, aka The Doo Doo Monster, aka Sexual Chocolate, aka Mister Smiley, aka Sofa King)