It breaks my heart to see something like that but it's nice to know there are people like you taking him in and taking care of him.
The best would be to have him seen by a vet, I don't know if there is a question of money but I know there are some vets that reduce rates and donate time and services to rescue cases. The reason I'm mentionning is that the dog would do better by either having the wound sutured (it may still be possible) or a proper dressing for maximum healing.
Whatever you do he does look in need of a good bath
You might want to trim the fur around the wound to help keep it cleaner. A daily application of antibiotic ointment (a light coat) would also be beneficial.
Good luck to both of you and keep us posted.
Glad to know he's in good hands