How Will Neutering Impact My Puppy's Weight And Personality?

Neutering him at 6 versus 9 months should not make a big impact on his final size.  By 6-7 months most of his bony development should be complete.  He will fill out after this time but will not grow dramatically.  It sounds like he is cryptorchid (testicles have not descended appropriately into the scrotum).  It is very important if he is cryptorchid that you get him neutered.  Testicles in the abdomen are at a great risk for testicular torsion and testicular cancer.
Neutered male dogs typically make better pets then their intact counterparts.  They do not have the urge to roam to find females.  They are less likely to mark their territory.  Dominance and aggression issues are much less common in neutered males. 
Neutering does take away some demand for calories.  So after neuter you need to cut back on his food and watch is weight and body condition closely.  Your veterinarian can assess his body condition and help you decide how much to feed the little guy.
Talk to your veterinarian further with your concerns about time of neuter and personality issues.

I feel confident that Ann knows what she's talking about.  From personal experience, when we had our dog neutered, he did gain weight.  To be fair, he was underweight when we first got him from the animal shelter, though.  When he started gaining weight we put him on a weight control dog food...didn't help...he still gained 3 more pounds.  He goes out daily and romps and plays.  As far as personality, he does seem less aggressive.