You could say that he has created a personal bubble for himself. At this point, he won't be able to trust people unless they can casually enter and leave his bubble as they please. He wont and shouldnt trust words, but instead should trust actions that can be verrified. When you do something for him, get him directly involved. If you recieve updates, give him a small update.
If there's no updates for a long period of time, give him a call and give him a status update. When someone has been wronged, he'll be cautious as you said. More information will satisfy his current predicament.
If it's a product you need to sell him, you can give him the back story. What do YOU profit out of it, and what does HE profit out of it. People like this (and me) will often ask themselves "Yeah, why does this person want me to do this so badly? What does he benefit from this?!". Depending on how skeptic he is, you could even talk about the business model if you have one.
In the end, to get someone like that, you need to feed him all the information he needs or wants. Let him confirm his trust in you on his own through validation and proof.
Avoid cornering him. This is something that people do very easilly. Preferably, you should avoid having him to do anything at all, but still inform him about what you did. You know, the steps you took to provide him with x. People like this will easilly fall back when they are being asked to do the smallest things.
Depending how recent he has gotten "burned" he'll be WAY more skeptical. For as long as the things you say are logical and can be confirmed to be logical by others. He should have no reason to make things hard on you.
Also, words dont mean much to him unless it's written down. If you confirm something to him, write it down in an email or something similar to him.
What I'm suggesting here goes quite extreme, so I'll let you decide on how far you wish to go. Just see him as a skeptical person, and you should be fine.