Hey there,
A date, how wonderful. I agree with Lyn, it is best to be upfront; your on meds so you are like all the other people with a disease that take meds whether it be a mental health disorder, endocrine disorder, respiratory disodrer ETC.
I don't know that I would tell this person on the first date as it is not necessary to share your whole medical history but when you feel comfortable I would tell him toward the beginning of a relationship that may continue on.
He is luciy to have met someone as lovly as you and I hope you have a nice time. Just be yourself and know you are a good person which he is lucky to have met. :)
Enjoy
Hugs
Kitt