My boyfriend (20 yr old) once a week will not be able to maintain an erection. however, in other days we have sex we have no problem. does he have ed?

Psychogenic ED: Probably anxiety related or psychogenic factors can play a role in his situation. He is still too young for organic ED. It is very common in young man and does not constitute a pathology

Something else: As your boyfriend what environment or interpersonal stressors he is experiencing before, during, and after that particular day of the week that distract him from having fulfilling sex with you.