I would start improving his immune system with diet, making sure his nutrition is good, and chiropractic. All will help a person's immune system behave better, and lower the chances that he will continue to succumb to strep.
My mom fought the good fight with my nutrition and then I ruined it all at 20 by having my second-line immune system (nasal passages being the first spot that germs are caught) yanked out.
Went from painful but ultimately not terribly bad strep to letting the germs go into my lungs (poor tonsils were swollen because they were *working* for me), and now I get SCARY illnesses when I get sick. I'd prefer the sick and wrong sore throats, quite frankly!
And good for the ENT for knowing you can still get strep without tonsils.