Oh man, I sympathize. I have the same problem searching for new soaps or body mists. 99% of them give me migraines or make me feel ill so of course I want to test them before buying any new kind...but in testing them all I just feel super ill because I've inhaled so many gross chemicals in one sitting. XD
It seems like it'd be risky/difficult to buy a scented product for someone else, since people's opinions on and sensitivities to smells can differ so greatly. Do you think your dad would be content with a gift card or something to let him pick out his own? Then you don't have to deal with the smell tests and he'll be sure to get something he likes.