I would say Python is, in the long run, infinitely more useful to know than AGK's version of Basic. Python is pretty easy to understand, but it's also used professionally in a lot of places so it's a good skill to have.
On the other hand, AGK's all-in-one IDE, broadcasting, online help and general ease of use might prove to be less of a hurdle for a beginner.
Me personally, having learned to code on Amos, then DB, then Blitz, and now AGK...I'm kind of stuck with only being able to code using such tools. I can't make heads or tail of C++, JS, or Python and don't even know where to begin to use these language, even the visual stuff like Unity and GameMaker totally flies over my head. I code just for fun so it's not a problem, but if I had to do it professionally, that would definitely be a problem.
As JasonPC said, I think the instant gratification of pressing F6 and seeing his app run on a phone/tablet might be the spark that keeps your son's interest going.
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We all got a chicken duck woman thing waiting for us