In my opinion:
When you exposed your cards before his chips were put into play, you invited him to
save his money. Yes, he said he intended to call you, but you'd not actually made your
play, i.e., put your money on the line.
If you'd moved your chips out, then any verbal comment by him would be binding.
The "muck" (n) is the pile of discards. Muck (v) means to discard. Merely exposing
your hand is not, I don't believe, the same as discarding, if you keep them in front of
you. It's not the brightest idea, but I don't believe it's illegal.
The situation turned into a pissing match because neither of you strictly followed the
protocol of the game. I know, "it's just a home game," but this shows ta go ya what
can happen when you get lazy.
The rules are the rules, for good reason. They prevent misunderstandings and disputes
when they're followed. Nobody can bitch about what you do if you follow the book.