Your husband probably has a higher resistance (or better health history) to whatever is effecting you and your son (since you say "HARDLY smell"). As far as the doctor visits and tests that you plan, be aware that the doctor is limited by being able to use only what the Food and Drug Administration has approved for use.
All diet soda should be avoided, any artificial flavor enhancer including Mono-Sodium-Glutamat (Natrium Glutamat) is a dangerous chemical, but putting them in baby formula is the saddest statement of social irresponsibility I could imagine. I once saw a bottle of diet root beer that had phenylalinines that warned about PKU, but in small print, so read everything BEFORE you buy.