Your son, like most people who get Lyme, probably had a bulls-eye rash and/or recalled being bitten by a tick. A bulls-eye rash is enough to diagnose Lyme disease.
Not only that if you start coming down with symptoms of Lyme right after getting bitten by a tick then diagnosis is very trivial. Those who don't have a bulls-eye rash or recollection of tick bites and don't spend a lot of time in the woods have non-specific symptoms such as fatigue and joint pain. Those are the cases that the diagnosis can very easily be missed.
Nowadays Lyme is so common around here that anyone complaining of any non-specific symptoms gets a Lyme blood test. 10 years ago that was probably not the case. A friend of mine went to the doctor with non-specific symptoms and tested positive for Lyme. If her doctors didn't run the Lyme test they would have missed it. They ran the Lyme test even though she didn't spend any time in the woods or remembered any tick bites because they now know to.
If Lyme isn't on the radar of doctors that is a huge problem for diagnosis of some cases. It probably would especially suck for someone who lives in, say, Utah who went to vacation in Maine and didn't recall a tick bite. Your average Utah doctor has probably never even seen a patient with Lyme disease and probably won't think to check for it or ask if you've traveled somewhere where Lyme is common.
Source: http://www.cdc.gov/lyme/stats/chartstables/reportedcases_sta...