How old is he now, that he is displaying this sort of behavior? There are different stages of development that they will show signs of fear, lack of confidence, and its just a matter of getting them through it without anything major sticking with them. Hopefully that is what your dealing with at the present.
Of course, each pup is different and I agree, 6 weeks is far to young to be seperated from mom. While I've never done it again, I have to admit that my Ivory, my first Newf, was that age when I got her. Of course I didn't know any better, and fortunately for me, she was a very outgoing pup. I had no idea that was her individual personality, and I had no idea how much she really carried me through those developmental phases. The only thing I know that I did do, was that I took her everywhere with me. I had no giant breed experience, and the only goal was that I knew she was going to be large and I wanted a well behaved girl. So we did obedience class after obedience class, because I was clueless and she loved it. I spent hour after hour sitting outside other classes watching and making her behave but be social. I played with her A LOT, we rolled around on the ground together like littermates, and when I told her it was enough, that was the end of it. She knew no stranger, because even though we lived with a cat, we both were each others best friends. She spoke volumes with her eyes, and it ended up being what got her anything she wanted, as she was always aware of people and things around us.
On the flip side, my girl Lu, who is now 12, went through a phase at about 6 months old where she was afraid of my dining room. It was very tramatic for her, and nothing bad had ever happened to her there, and what made it harder was that is the central room in my house. You have to go through it to get to any other room and she ate about three feet into the kitchen which opens off one side. It was carpeted, so that wasn't the problem, she was just to scared to go through it and when I walked her on a leash she would bolt to the other side to get out of there. It lasted for a week, and it was gone as quick as it came. No explanation except it was just a developmental phase.
So to cut this post short, socialize, socialize, socialize....Its okay if he doesn't know he's a dog, we won't tell him, and don't lower your expectations of him just because he didn't have the extra nurturing from mom/littermates. Of course having that extra time, would have been best, but he can still be normal, or crazy, depending on who he is and how he is exposed to things.