Both of these are great options.
Personally I'd take him to the Morris Bridge Road section and stick to the main trails.
The yellow trails at Santos would be good as well.
If you go to Boyette, take the two track to the first fireroad on the right and follow it down till you get to the Quads. If you feel spicy after that try Palmetto Cable and East/west Meadows.
At Alafia the River loop and Sandpine would be fun. If he does good on those, then look at rock garden (stay off the black loops!)