Oscar was a cryptorchid neuter as well - one of his testicles was hiding under his bladder. You'll definitely have to have your boy neutered - unfortunately you wouldn't be able to show him with only one descended testicle anyways. At 6mths, chances are if it hasn't descended yet, it probably isn't going to. I doubt that his fall had anything to do with any of this - he's probably just a little guy - he could also be a late bloomer - dogs have growth spurts at different ages and you may find he'll start to gain weight again in a couple months. Oscar was 2.8lbs at 7mths when he went in for his neuter (is now 3.5lbs at 11mths). The surgery is more expensive as it is a little more extensive (two incisions - one at the scrotum and one in his abdomen along his penis - and they have to basically 'hunt' for the hiding testicle). Oscar's neuter was just under $400 (but all vets are different). For health reasons it is definitely better to have him neutered. You can do a google search for more information, here is a link that I found:
http://www.peteducation.com/article....&articleid=897
Hope this helps