What the vet thinks happened is that the fleas have sucked so much blood from poor Maxx that he's gone anemic. He's lost a serious amount of weight, right now he's 8.25 lbs (normally he's 13-15 lbs), so that shows that the FeLV is back and active. It seems that the anemia caused the FeLV to come out of remission, and what I witnessed was a seizure.
I'm to keep my eye on him, make sure he's eating (which he has been since the seizure), and if he stop being happy and perky, or if he loses more weight, or if the seizures start becoming common, I'm to take him back and we'll decide if he's to be put to sleep. One or all of the above is very likely to happen in the next two months.
So, the news is only somewhat good for now.