Have him put it in the middle front, and about middle rear (maybe 1-2 cogs larger/easier). Follow him on a ride and tell him he's not allowed to shift. This will give you a rough starting point. SS drivetrains are more efficient, and he will get stronger, so you may want to go 1 tooth smaller on the rear cog once you figure out his optimal gearing.
I grew up riding miles and miles on a SS BMX bike, but that was too long ago to remember the gearing. Kids that age will adapt to pretty much anything though.
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