As I pointed out Tyler Hamilton listed thyroid supplementation on his training diary, wherever he published it - his book, wherever. I remember seeing it, as I am hypothyroid, and have been taking thyroid for quite a few years.
I can tell you first hand, that if all the doc is doing is running the TSH down to the lowest "normal" level, he's not doing anything to get fashed about. Taking the TSH LOWER would be a concern - in that condition you can experience heart palpitations, high blood pressure, overheating, bone density loss (not quite so obvious as heart palpitations!).
If he's just saying the "trigger" point is a little lower than the ordinary medical consensus, I certainly wouldn't get fashed about that, either. I tend to agree with whoever said in the article that most endocrinologists, ftm, most gp's, don't have a clue. Actually, I'm not QUITE that dismissive of medical knowledge, but my experience has taught me that the medicos are rather more fallable than they would like us to believe. Hypothyroid can be difficult. The biggest symptom is just being tired. Dog. Tired. But if you overcook and are taking too much, you can tell pretty quick, with fairly small amounts.
I agree with the OP that he should have his son tested. I was given a type of medical treatment, the byproduct of which was killing my thyroid - but before I started the docs tested and said I was low. Could have been that way for decades, and could have been why I often suffered from fatigue, and just didn't know. The tests are done from a simple blood draw - you get the results back in a couple of days. No problem.
Just to restate, taking someone in the "normal" range - but slightly on the high side - and reducing their TSH, within "normal" ranges - imo should not be a concern. I would say it ain't doping. It IS trying to be normal when some part of your body is malfunctioning.