Husband gets severe ear infection every 3 months in same ear. so severe his ear swells shut and causes him a lot of pain. what could be causing this?

Otitis externa: The fact that you tell me that his ear swells up makes me think this is an external canal infection, as opposed to a middle ear infection. This is caused by bacteria invading the external ear canal ( like a swimmers ear). He can try putting white vinegar in his ear once to twice daily for prevention of these infections.

Recurrent otitis: I would want to make sure he isn't diabetic which suppresses immune function and that he is completely well to avoid recurrence. Treatment with acetic acid vosol (acetic acid and propylene glycol) afterwards might help prevent recurrence. Has he seen an ENT specialist?