@scottwilson I think a behaviorist could definitely help the dog but also you guys as owners. A lot of times people don't recognize some of the more subtle tells that a dog is getting uncomfortable. I'm not blaming you or anything, so please don't think that, but saying their isn't any warning is most likely an incorrect statement. There are a lot of warning signs dogs give off, more than just growling and raised hackles. A behaviorist could help you identify those as an owner while also working with your dog.
We had family friends growing up that would just put their dog in a kennel in their bedroom when people were over because they knew their dog did not do well with new people/lots of people. Not saying you have to do that, if you don't feel you can work with and handle the dog's shortcomings then that is fine. The rescue can try and find a better suited home with owners that live a lifestyle more conducive to the dogs/have more experience with his type of behavior. Just saying there are options before jumping right back to dropping the dog off at the rescue.
It stems down to your comfort level, the amount of time, energy, and money you're willing to invest, etc.
Like someone else mentioned, you should also take him to a vet to see if there's anything health wise that could be a contributing factor to his behavior.
Also, it is not the best to stick your hands in a dog's face. Most people do that to "give them a chance to sniff them" but it shouldn't really be done. Your partner definitely needs to be aware that this is a serious matter though...5-6 stitches is serious