S.F.
I went through this with both my sons. There wasn't much I could do while they were at school, except pray the smell wasn't too bad. But, at home, any time they smelled bad, they had to shower and change. They finally learned to wipe better because they got tired of taking several showers a day. Plus, I made them do their own laundry (with supervision) so they could see what I saw. Gross!!!
You could also make wet wipes available to him. Sometimes poop is sticky and all the toilet paper in the world won't get it off. They do have septic system safe ones available, if that's a concern. Again, they're hard to lug around school, but they can be put in a baggie and then put in a paper bag, if need be.
Sadly, the best fix is discovering girls...and realizing that smelly boys get zero attention from the girls (and smelly reputations take years to live down). My eldest "recovered" from his "soap is evil" phase about 2 seconds after this exchange...
Son: Mom, how do I get girls to like me?
Mom (and dad shouting from his office): TAKE A SHOWER AND USE SOAP!
Problem solved. Now, we just get to deal with the drama of girls in our lives. Sometimes I'm not sure which is worse... <g>
Good luck.