Do a jokey pantomime booo. This highlights the issue without it getting heated. Or walk out the room suďdenly when he starts to make him aware. A lot of the time people don’t realise which doesn’t make it any easier and still gripes me they have no awareness when they do it and how it may afect others.
Make a joke out of it like “What you doing that for?” Or “Is that fun?” This can often help.