One of the best things you could do would be to simply ensure that he takes a full hormone pathology panel and tell him that you took an antiandrogen medication (don’t detail much more about it- be deliberately hazy- with some doctors if they find out it is propecia they will dismiss it)
LH
FSH
SHBG
Total Testosterone (free testosterone if possible as well)
DHT
Estradiol
Prolactin
If possible
TSH
Free T3
Cortisol (24 hour urinary if possible)
If something is amiss it should appear on the test results one way or another.
If you ask your doctor for the test results and reference ranges that come with them and post them in the relevant forum then people will be able to comment upon your results and give you their opinion of them.