treats and trust, mostly the latter
try to find his "heaven" treat/nut/biscuit or what ever he adores, hold it near the opening or top, but don't force him to step up, let him come to you, take the treat, go away, come back and stay a little, then move the treat to your other hand and use your main hand as a bridge, just let him step up, don't move, get the treat, slowly, slowly gains trust
he's young and your very big and scary, it will take time, but just go slow, trusts takes a lifetime to build and can be lost in an instant, so dont force it or rush it.
you could also try a target stick if he's really nervous, but being hand reared, shouldn't be that scared of hands.