I think you may be suffering from a serious case of what I've seen termed "impostor syndrome"*, but odds are that your son really is that smart of a cookie. I think a poor test result doesn't prove a kid is not smart, but a great test result isn't the result of chance.
I do think the tester does make a lot of difference. I think this might be partly because they are allowed some leeway, depending on the test and subtest, on how they score the verbal items especially (0, 1 or 2 points, that sort of thing). But I think that it's far more likely that a particular tester would give higher results than another one due to establishing good rapport with a particular kid. That's how it was with us.
I don't attach a lot of significance to milestones, and never have. No matter what, your son must be way ahead of most age peers in ways tested on the IQ test, even if he wasn't as far ahead on achievement tasks (as tested).
I would try not to sweat it, and just be sensitive to what your son seems to be ready for and interested in. He's bound to be unique anyway.
* ETA: BTDT
