Nobody, least of all your boss, will complain if you grow the companies revenue without significantly expanding on your cost base.

I would suggest thinking of this as a challenge, see if you are up to it. Grow it in a way where your boss is not taking on additional risk from scaling up prematurely. You mention the reliance on a few key customers, on this front you can also de-risk.

Also he does sound like he has his head screwed on. Ask him for more detailed feedback. I would imagine he has looked at your plans for growth and don't see them as a good idea because of X, Y, Z. Figure out what these are and take the criticisms on board.