
If he's fully erect he's got too excited too soon and can't enter her like that. Dogs only have the end bit showing before mating and when they get the right spot and enter the bitch the sheath is pushed back.
The problem is probably that he's rubbing against her and getting too excited without being in the right place. With a keen inexperienced dog like that I would pull him gently away and let him cool down for a few minutes then try again. If you kneel at the side of them and reach your hand under the bitches back leg when he mounts her you can gently guide him into the right spot. Don't get hold of him too tightly or you'll have the same problem! I use one hand, my pinky feeling where I want to aim and two fingers guiding him. Once he gets the right place you will both know it, he will almost lift his back legs off the ground - climbing the ladder some people call it. At this point move to the back and clasp your hands round both their hindquarters as a maiden bitch may struggle and a maiden dog might back out - so hold them together till you're sure they've tied.
How tall is the dog in relation to the bitch? Matings are easier when the dog is slightly taller, if not you may have to improvise with sloping ground, things to stand on etc.