I slightly disagree - do say "arseholes". Its a word designed to help your boy remember your advice - and its accurate. Think of it as an aide memoire (sp?)
Something I've used in the past ...
"unfortunately the world is full of arseholes and sometimes you will encounter such people in your day to day life. There is little or nothing that you can do about this other than chalk it down to experience and move on, secure in the knowledge that you are a...