No! Don't ...unless you out yourself.
Your in law asked a favor not expecting for you to find out. He will be horrified if he knew you found out. If it's important to you.. you need to take the step and come out to him first and not tell him you know. If he wants to tell you, that will be his chance. THEN you can mention you thought so.
Still take it slowly, feel him out. Someone mentioned it might be a trap so there is a risk.
If the risk is too great, forget all about it. The closeness you share could be a detriment.
My 2 cents
CAT
Not about you...about him
I second all the girls that caution you not to say anything about it. The only way this is remotely acceptable is if you come out to him first and let him decide to come out to you.
Hope it goes well.