Can see Thon sliding to late 2nd Rd especially if his combine isn't good. Going to playing a couple of year in the D-League (maybe NBL?) regardless.
It's always a chance, but I'm tipping he goes somewhere mid-late first round.
I reckon teams will have pretty low expectations for him in the combines. They are not gonna be looking for the guy to be able to compete with the guys with college experience... he is coming straight from high school in Canada.
What they are going to be looking for is a strong work ethic and effort- and that he physically looks similar to advertised. He has all the physical attributes that NBA GM's will be drooling over. (except for the bulk - but look at guys like KD and Kristaps, and he doesn't need to put much on, just enough to be a bit stronger).
If he looks good from that side of things - I wouldn't be surpised if he goes in the lottery... he definitely will not be the first untested guy to be taking high based purely on potential.
If they are worried about the effort side of it... then yes, he may well drop to 2nd round, or go undrafted - but that would very much surprise me.