Does he thrust when you jerk him off? If so, there is a small window of time when you can quickly slide his sheath back that starts when he begins thrusting and stops when his knot grows a little bit. Ideally you'd time pulling the sheath back to just as he starts a new thrust. Be careful because after his knot grows a bit it will be too large for the opening of the sheath, and trying to take his penis out would hurt him. Some dogs are harder to pull back the sheath than others but the best way is taking his lead and trying to mimic what would happen during natural dog breeding.
Also, I suggest ensuring that he wants to be jerked off. Dogs can resist having their sheath removed if that is not what they want, and that could explain the trouble. Dogs get hard and even thrust mostly involuntarily so that doesn't guarantee he is 100% on board. Look for signs of discomfort. For example, if you gesture that you're going to jack him off, does he stay still or move away?