What helps my kids is we start them with very specific instructions. For a boy, roll the TP to the floor (no further), hold the top with one hand and rip off with the other, crumple LOOSELY (this takes practice and is a good thing because they can see their progress in learning the skill), reach behind, wipe firmly from front to back, look at TP to see how clean you are, repeat as necessary. During training, if they need it, I will step in and physically help them figure out how hard to wipe to be effective.
There are a huge number of skills to coordinate in this one "simple" task (not unrolling the toilet paper when they 'rip' it is one we've had the most trouble with)!
Good for you for seeing that you need to start him early!
Unless your school offers wet wipes, be sure to consider whether you want to use wet-wipes as an initial training and move to TP, or just start TP from the top. (Our kids' school has urinals in the boys' bathroom, and we were requested to train our 5 year old for that, which was a bit of a surprise, but luckily his dad is still very active in the kids' lives ;)!!)