Moxie, we're kind of in the same predicament. Our youngest who's six now had horrible ear infections from birth. She got tubes put in at a year and a half old which both came out earlier than the doctor wanted. In fact, she still has to wear an ear plug in one ear when she swims or showers because the hole from the tube never healed. Anyway, she failed her most recent school hearing test. She doesn't have a problem with infections anymore, but she does get colds and she has allergies both of which affect her hearing and I think she had a cold at the time of the test. The school is going to test her again in the spring and if she fails again we'll be taking further measures, but nothing at home raises any red flags for us nor does her teacher have any concerns.
The ear infections and tractors could very well have compromised his hearing so my only advice is to do whatever those instincts that we mothers have tell you to do.