I would also recommend getting OUT of the house to meet other dogs in neutral territory.
It sounds like you're making good progress, but he's still not interested in sharing his own home with strangers. I think he needs to meet more 'strangers' (dogs and people) out in fun places - like strip malls or ON-leash parks where you can observe people/dogs from a distance and make sure he's comfortable just observing before you take him in closer to actually meet or interact with the strangers.
In the home, I would work on just a very strong "place" command... and make sure he knows that's a safe space - where strangers will NOT approach him, where he can observe the new people without fear of needing to protect himself. Strangers should come in and IGNORE him... and randomly maybe toss him a treat without actually looking at him or making eye contact, so just them being in the room makes treats "appear".
Once he's comfortable observing from a distance and can remain calm on his 'place' during visits, you can have the strangers try to entice him to come to them, with treats, baby talk or toys. If he doesn't engage, go back to ignoring him. If strangers sit on the floor, versus up on a chair, that might help him be more curious about the whole thing, too.
I would also say when he barks and growls at new people coming into the house, don't get angry or mad at him. Don't yell "NO" at him - he's trying to tell you he's uncomfortable, and asking for your help. I would calmly leash him up and remove him to his crate or a back room to let him know you don't need his "protection", and YOU will protect HIM by removing him from the uncomfortable situation. If he's ok with that, you could leave the crate/room door open, so he can choose to come check out the new people on his own timeframe.
Anything and everything you can do to boost his confidence will help. Teach tricks - so you can praise him for doing as asked - take him on walks to new places to explore and really encourage him to be curious about new stuff (people, places, dogs, objects, etc.). It might take time, but he'll come around as long as you're consistent, patient and generous when he's done something you like.