Yes, an ANTLR grammar is a formal system. Based on this, you can just declare victory with a straight face.
He probably wants something other than ANTLR syntax. And, rightfully so, the annotations in the ANTLR grammar for building trees, etc, just confuse the issue, as do the various lookaheads. So, in your shoes, I'd remove all the annotations, and probably recast the grammar into IEEE EBNF (See http://en.wikipedia.org/wiki/Extended_Backus%E2%80%93Naur_Form). And you can point him to that web page, and declare you have a "pure context free grammar".