[Por_sha911]

I imagine that some "expert" (meaning goofball brother in law) told him not to get in the middle of your legal battle.
Remind him that without the letter you don't have a case so you will have to deduct from his bill what you would have gotten back if you could prove you got bad goods. Tell him you made a deal and expect him to keep his end of it.
If he won't help, then you can either suck it up and go elsewhere with your business or tell him that you will tell the world that he's a creep and see if his tune changes.