If that's the case, well I can understand your position then.
You definitely need him to state his criteria in writing, and if possible have him provide a drawing along with specifications based on his requirements.
After he has provided this information you can reply with a request for information which address and list your itemized concerns. Also per each concern, make you recommendations very short and in a direct manner. Do not list price for these items.
Have him reply again in writing. You will then make a revision to you original quotation or estimate based on his input.
Take his final decision and a simple drawing which you will submit for plan check and review at the city where this work is to be done, and hopefully with a stamped approval you can obtain a permit for the HVAC work based on his criteria.
Have him agree to his own design and accept full responsibility for the performance of the HVAC system.
Hope this helps.