
Assuming they are both the same breed (you never know these days

) this is why there are Breed Standards. Perhaps if he's that short in the leg, and she's taller, the two are not meant to be mated together? Having lived with my main breed (Bassets - short legs, long backs) for years, mating wasn't easy, sometimes. We'd have to sit with the bitch across our knees and guide the dog in. Some dogs didn't like to be helped however. I had a male who would help himself and backed off if I tried to interfere, so perhaps when she's right on, nature will prevail without you helping. Many times, a bitch will help too .......
Can you get an experienced breeder involved?
ps I had a good friend in the breed who we took our males to, meeting up with bitch and owner at her place. She was able to get a mating done, even if with another of mine, it was always a slip.