My daughter sounds exactly like your son. She had one round of tubes at age 17 months, and then, since the infections only slowed, but still happened approx 1/month, her second round of tubes and her adenoids removed at age 2 years, 3 months. That was a year ago. She still gets the occasional infection (maybe 3 in the last year?), but not nearly as much as before. However now they clear up on their own, or with the use of Ciprodex ear drops that were perscribed by her ENT. We have tried other things, changes in diet, chiropractor, etc, but hers are mainly a result of small ear canals. Maddy's ENT has us contact her any time she has an infection, mainly to track them, but occasionally to see her. They really have gotten better since the second set were put in.
You may want to discuss larger tubes with his ENT, and adenoids.
Good Luck!