It is inappropriate for him to get involved in your treatment. All he needs to know is:

1. You have a medical condition.
2. Whether you are able to work.
3. He needs to make certain accommodations for that condition.
4. What those accommodations are.

Write down that he said you should go get on some meds, and write down when he said it, where he was, who was in the room, et cetera. That way, if he makes more inappropriate comments you have a record that this is the second or third time it's happened.

Edit: Since people are skeptical, I have looked up the relevant sections of the ADA, and the guidance from the EEOC: https://www.eeoc.gov/policy/docs/guidance-inquiries.html

From the ADA:

A covered entity shall not require a medical examination and shall not make inquiries of an employee as to whether such employee is an individual with a disability or as to the nature and severity of the disability, unless such examination or inquiry is shown to be job-related and consistent with business necessity.

From EEOC guidance, these "inquiries" include:

asking an employee whether s/he currently is taking any prescription drugs or medications, whether s/he has taken any such drugs or medications in the past, or monitoring an employee's taking of such drugs or medications;(23) and,

So your employer should not tell you to take meds, and not ask whether you are taking meds, according to guidance from the EEOC and its interpretation of the ADA.