On the Spot,
The rating can be revalidated at any time during it's validity provided you meet the requirements.
Your CFI was wrong!
If you decide to revalidate outside the last 3 month window the your rating will now run from 2 years from the date it was signed and you will loose the ramaider of your current validity period.
The validity of the medical is a totally separate issue to the rating and is outside the authority of the CFI to comment on unless they are an AME or you are trying to exercise your licence and rating without a valid medical.
Regards,
DFC