Are we supposed to believe that a guy can be completely incapacitated and put in a coma for YEARS from just a knife wound?

i was also confused

I thought maybe they gave him something through the oxygen mask, too? Because they wanted him to stop killing people and as soon as the mask was off, he woke up.

Enough blood loss can can cause someone to go into a coma. I assume that because the mother didn't want to draw attention to the fact she stabbed her son, she didn't call an ambulance or any help, and as mentioned in the show, the father wasn't picking up the phone. So... maybe he was bleeding out until the father came home?