Is this bipolar? could stroke make this worse? advice? read more info..?

my bf (17y) can be happy one minute and sad or angry the next. we used to talk on the phone alot, he starts out cheerful but i say something and he takes it wrong suddenly hes sad and mad at himself over dumb things. i always have to watch what i say because he takes everything wrong and he thinks everything he does makes me sad....he recently had a stroke because of complications with heart surgery. i would cry alot because it hurt so much to know everything hes going through. he isnt the type of person to cry he wuld even feel bad because he cant cry wen something bad happens but i would hear him on the phone..crying because hes scared of more surgeries or another stroke he would say hed rather die..i cryd so much because i culdnt comfort him and he said he blames himself for making me cry. before the stroke he would have some mood swings but now they seem worse every time we talk by the end of the day i say sumthing not bad but he always changes this last time we talked he yelled at me and broke his phone so he couldnt make me sad anymore while hes in the hospital. he also seems more agressiv towards family and a little towards me. his sisters have mentioned they think he is bipolar jokingly but i think they wer also sserious. could all this be from the stroke or depression or bipolar?