
What will happen to his coat over time, is that he will grow a profuse wooly undercoat, loosing that typical snooth and silky top coat. The top coat usually becomes dryer, and more prone to tangle. The main problem is that the dog becomes very hard to groom, and the coat gets matted, and doesn't lie right.
There is a slightly greater risk of a retained teste becomning cancerous, but themain problem is that you wouldn't see that there was a problem like you would with an external one. Actually testicular cancer is not very common, (unlike mammary cancer in bitches), and a male dog is more likely to develop prostate problems. Your vet could have removed just the reatained testicle, and left the other, this would have helped with the negative coat changes.
Now that the deed is done, you will need to keep on top of his coat more, and have him clipped regularly so he doesn't look like a walking hedge.
Hope that helps.