I had a 4 month old chihuahua that weighed 1 1/2 lbs and his testicles never dropped but I ended up fixing him when he turned a year old, he was then 4 lbs. and I could have probably done it sooner. I don't think they will come down for you especially at 6 months old. My vet told be to massage his testicles down and I actually tried that for a few weeks right when I got him, needless to say he loved it!!
But it was all for naught!! I would just wait and see and let him get bigger and then fix him, it is true that they have a higher chance of testicular cancer when they are undistended. Good Luck with your little man!!