How do you tell your brother-in-law to *******?

Don't get me wrong I respect my brother-in-law, but I think he has some control issues he cannot control outside of work. He was a frat boy in college, but he is grounded as well, but everytime I try to be helpful and give suggestions about something he is working on he tones in a condescending way and riducules my heartfelt suggestion in the matter. It has gotten so bad that I refuse to help out anymore for the sake of another condescending put down when he feels my suggestion is not up to his knowledge. I mean whatever happened to constructive criticism once in a while? I tell you one of these days my temper is going to flare and I am gonna put him in his place real quick and that will probably cause indefinite damage to our relationship and to me and my sisters. Any suggestions how to tell him to **** off sublimally without actually saying the words, "**** off" so he gets the message?