My boyfriend has asthma & randomly gets moments where he can't breathe, even if hes just sitting there. What could be the cause?

Poor control. Of his asthma. Compel him to see his physician to get better control of his symptoms.
Asthma can. Asthma could be the cause. It would be easy to find out, if he takes his rescue inhalers and the symptoms get better then it is likely to be asthma. There is much more to this and he should be talking to his doctor. You need to take into consideration what other medical problems he has and what other symptoms are noted.