S.,
I don't know that you can reduce the size of the tonsils that are enlarged. However, if the issue is that you're trying to avoid a tonsilectomy, just opt not to do the surgery. Unless, of course, the issue is that he has a really bad case of strep where the bacteria is eating away the flesh. I had a friend who had permanent perforations in the back of her throat as a result of a case of strep.
I had enlarged tonsils as a child and had chronic sore throats. My pediatrician, at the time, recommended that the tonsils be removed, but my parents opted against it.
Today at 34, I don't have any problems, lest the occassional sore throat that accompanies a cold. I'm actually glad that my parents didn't go ahead with what the doctor suggested. In fact, when my mother had a tonsilectomy in her 30s, she developed allergies to all kinds of things (i.e. grass, chocolate, beans, milk). I think tonsils are really important even though we don't completely understand their purpose. My tonsils still seem kind of large to me, but they aren't problematic.
If you do find something, let me know. I would love to hear of suggestions should my daughter one day develop my chronic sore throat.
Good luck!
E.