Spend time with him and be a leader. What works for me is just hanging out, grooming and not expecting too much for the first few days. You don't give much info here, like age, training, what you want to do with the horse, Is this your first horse?
The time it takes to develop a relationship with a horse depends on the horse. Some horses attach quickly...some take more time...and some never give a hoot.
If you ever find yourself in a fair fight, it's because your tactics suck. ~ Marine 1SGT J. Reifinger