I'd say nothing of him behind his back I wouldn't say with him standing right in front of me.
[It's teasing all in good fun, and Merlin is good enough a friend he'd know to take it as such. He'd never insult Merlin, ever, he values him far too much for that.]
Besides, one can bear armor well and still be undeserving of one -- Merlin bears none, but would deserve one more than most of us combined.