Don't worry ah... your BB still very young, only 31 wks. They'll consider your BB to be 慢性肺病 (chronic lung disease) only if BB still need O2 after 36 wks.
My BB was unlucky to catch a strong 肺炎at 42wks (just when doc told me to get ready for him to go home). His O2 increased from 1/8 flow to 2.0 ah. I was very very unhappy because I waited so long for him to recover and then sick again.
He was finally home after being in the hospital for 6 months and still need to use O2 24hrs at discharge. But I'm very happy to see him smile and to talk back when you play with him. The road may not be easy, but be strong for your BB. +oil la!!