Definitely start training him how to do it now and just follow up to make sure.
My youngest took a LONG time to do it right because she would want to be done with it and go on to playing. I drilled it into her head to wipe at least 3 times and always look at the paper to see if it's clean. After the 3rd if it's still dirty, flush and start again. I had to set up an incentive of new "big girl" silky panties once she could do it on her own for one month (we had a jar and glass stones to keep count.) This was when she was 8 that I finally found something that worked! I even had a sign made up for the bathroom with each step.
My eldest had it down cold pretty early on, like by 4. So I couldn't fathom why my younger daughter just wasn't catching on to the importance of a clean tushie. Ah, the joys of parenthood!