I always yell out "on your left", sometimes a quick "left" if I am suffering, or if there are multiple groups of people I am overtaking and too lazy to say it all, and sometimes, I mix it up and say, "coming by, left hand side". I used to just scoot on by, and not say a word, but it freaked out a few people I noticed, so I started with the vocal warnings. If someone ever said some crap like that to me, I would have responded depending on my mood with "harden the f__k up!", or "keep at it you'll get faster" as I dropped them with a quickness. It's not your fault he is slow. I was just thinking how I have never been passed to know if anyone else signals. The good thing is he is on the bike, starting to get in shape. Speed will come later.