How do I teach my son to wipe?

[HoleySock]

Just as the headline says really. My son will turn 4 in August and starts school in September. Potty training went great just before his 3rd birthday and he's been dry and clean almost constantly since - occasional accident when he's been distracted with play but I can count those on one hand in the least year.
I'm still wiping his bum for him after a poo. In all honesty it is somewhat convenience because he also has a two year old sister and a four month old sister and usually when he uses the loo it's easier to ask him to wait than to deal with poo everywhere but I KNOW he has to be able to do it in September so I want to start teaching him,

In the past I've asked him to pay attention to where he feels me wiping and given him the paper/wipes to try himself but he doesn't even get close. Are there any phrases or techniques I can use to help him? He's very bright and communicative and I know he will get it if we use the right technique and stick to it!