Imho, at this point just be there for him as a friend. You're not going to prove anything to him regardless of the strength of your argument. I'm willing to bet that less than 5% of bal teshuvahs (both ffb and not) made their life changes based purely on being proven that yidiahkiet is legitimate.
If you are a true friend to him and are there for him regardless of his level of observance, you'll have plenty of opportunities to be mikariv him over time.