It's never too late to socialize him. Socialization is VERY important and should be done right away. I started socializing my puppy when she was 9 weeks old. She is now 4 months and very easy going with other dogs. I don't have to worry about her possibly attacking or acting aggressively towards another dog. I will continue to socialize her, even as she grows older.
I fully agree with everything themacpack has said. The thing with training is that you have to be consistent.
You can't just attempt something once or twice and then give up because it doesn't appear to be working.
If the "quiet" command isn't working, then try something else. If someone comes to the door and he starts barking, get him to focus on you. Ask for a sit, lay down, whatever, and then reward [praise or treat] for quiet
behavior. You can eventually put the "quiet" command in there as well. Make sure you aren't rewarding him for barking though. Give him a command, and then reward once he performs the command.
How often is he getting exercise? I think you need to change things up a bit with him. Go for longer walks in different areas. Take him to a dog park [once he is a bit more socialized and is able to act politely around other pups]. Toss a ball, practice tricks, etc. Don't be so routine because eventually he may get bored. Dogs need to be stimulated. A majority of bad behaviors are because the dog isn't getting the mental exercise that it needs.
Here is a good website that explains what themacpack was originally saying when it comes to the barking. Barking: How to Get Your Dog to Quiet Down : The Humane Society of the United States