Husband Has COPD and what to look for to bring him to hospital

My husband is 47 and is diagnosed with emphysema.  I don't know what stage.  He doesn't tell me much.  He went to the doctors last week and his pulse ox was 85 then 88 when checked a few minutes later.  He continues to smoke cigarretts and pot.   He smoked both since he was 13 so 34 years.  He drinks and really doesn't take care of himself.  I should probably go with him next month to his appointment.  He's always out of breath and complains about tightness in his chest alot.  He is 6 feet and weighs about 135 pounds.

When he was 30 he had 3 spontaneous pneumothoraxes and he had a surgery to stick his lung to his chest wall.  In his other lung he gets small spontaneous pneumothoraxes which don't require a chest tube once in a great while.  

My question to you is what should I look for on when to bring him to the ER?  He doesn't do much.  He takes naps often.  He has started walking with me for about an hour a day.  He is not on oxygen.  I'm not to sure what to look for.  I know he needs to quit smoking and such but he has to do that.  I just don't want to find him dead at home.

Sorry for being so blunt but just wondering what to look for in case I need to call an ambulance.  Thank you.