sunnysukh5

He should enter before the K1 Visa expires. His medical examination should be valid for a year when filing for AOS.

Visas normally expire up to 6 months after the medical exam was completed......sometimes, they expire sooner.  I have never seen a visa expiration date later than 6 months after the medical exam was done......He must enter the US prior to visa expiration.

normally the visa expiration date is the date when the medical expires (6 months from issuing date)

The validity period of the visa is constrained by the validity period of the medical exam.
As long as you enter before the visa expires, you're fine. But don't expect it to be valid until Sept. 19.
 
See 9 FAM 302.2-3(C) https://fam.state.gov/fam/09FAM/09FAM030202.html
 

The visa expiration date is tied to the medical, but the embassy follows the recommendation from the medical facility who has the leeway to indicate what date they will use. Anytime you have additional testing it influences the expiration date. 
 
IMPORTANT NOTE:  Medical examination results are valid for six months in normal circumstances. But if you have certain medical conditions your examination results could expire in as little as 3 months. In any case, you should not have your medical examination until the NVC notifies you of your interview appointment date or you are near your interview appointment that you scheduled.  
https://travel.state.gov/content/travel/en/us-visas/immigrate/the-immigrant-visa-process/interview/interview-prepare/medical-examination.html

@Greenbaum ok thank you. Appreciate you taking the time to respond.