Hello~I know this was posted in September, but for the sake of others that might read this, I will give my opinion. Your aches and pains could be the start of a cold or virus, sitting under the a/c could certainly bring one on. Also, some arthritis can be flared by cold air. Do you have fibromyalgia? This also could be a cause. If it continues, then seeing a GP would be the next course of action to take.
I hope you found a cause for the issue and that you are doing well.
The cold air could weaken his immune system only if he got hypothermia or frostbite from it.