The best way to say this is probably:
You are much more trusting/trustful of him than I am.
This only implies that the listener is more inclined to trust "him" than you are.
This is not the exact same thing as "the benefit of the doubt", which implies that while something bad did - at least probably - happen, one assumes the the explanation involving the least malice - until proven otherwise. This doesn't always mean that you don't believe that someone did anything wrong, it can also mean that you assume they either made a mistake, or had a good reason/explanation. The concept can entail "trusting", "forgiving", and/or "understanding".
In this situation the best word depends on the exact circumstance. For instance, suppose a manager at a grocery store sees an employee eating some food. A trusting manager will assume they paid for it, and not bother asking. An understanding manager will say something such as "I had better not find out you didn't pay for that" in a pointed tone, giving the person a chance to go pay for it now if they had not. A forgiving manager will find out for certain, but only give the person a warning "you had better not do that again".
All of those cases are giving "the benefit of the doubt" to a differing degree. Otherwise the manager would check the cameras, and either know the employee was innocent - or fire them.