Is it rude to add onto someone else's tip?

Recently, my old boss took me and another former colleague out to lunch; when the bill arrived, he insisted on paying and we thanked him profusely. But after resuming our conversation, I noticed he'd left a shockingly low tip for what was excellent service.

 

I didn't know what to do. Obviously, I didn't want to appear ungrateful for the generous meal he'd just treated me to. Worse still, I didn't want him to feel I was criticizing him, or judging his tip. But I felt really bad for our waiter, ...