I agree, he should be working for you and use the wire you specify, even if you have to buy it yourself.
The only possible objection I can see him raising is that it may have to do with a building code issue, which may specify flame-retatdant or some such wiring for the speakers and/or other low-voltage wiring. In such case, I cannot see him objecting if you provide him with a code-compliant, heaveier gauge substitute.
So, it may behoove you to check out your local building codes and act accordingly. Perhaps a firm discussion with the general contractor may be in order. FWIW, you can't go wrong using 12 guage wire, even though it might be overkill in some situations.