This is Dr. Christian. Welcome to JustAnswer. A few questions will help me get a clearer picture of the situation:
Thank you for the information. The best chance at healing is surgery but if you are reluctant to take him to your vet then are some things you can try to help the situation. These can be difficult to heal due to the weight bearing down on the area, especially in a dog this size. It will be vitally important to keep him quite and minimize activity for 10-14 days or the cut won't heal well. Here's a plan of treatment you can try:
If the area becomes reddened or develops discharge then have it checked by a vet immediately. If you have more questions please let me know.