What temperature should I keep my thermostat for air conditioning during the summer for child w asthma?

I think my son is showing signs of asthma. The dr won't diagnose him with asthma because he was wheezed once. Right now he is really congested for about 1 week, but he has been coughing for a month. Right now Im trying to do everything to eliminate any triggers and/or allergens that can cause an asthma attack. I currently keep my house at 77 degrees but I think I should have it lower but not sure at what temperature. Thanks