German Shepherd dog

There is still a possibility the testicle will drop.  You should not get him nuetered at this point.  A nuetered dog can still compete in Schutzhund,. If for some reason his testicle does not drop,  I would wait until 18mos or older to nueter him as hormones play a big role in proper development, both physically and mentally. 

When is the approximate "deadline" for testicle descention?

Eva, I do not believe anyone can give you a "deadline" for testicles to descend.  I've have *heard* (warning - second hand info) that a dog as old as 9 months can have a testicle drop.  From my own personal experience, I had a male pup who at 15 weeks had only 1 in the sack, and the breeder offered a replacement.  Is that too early to make a decision?  Maybe, maybe not. 

In any case, my vet recommended the dog have the undescended testicle removed as it would be more prone to testicular cancer.  However, there was no time table.

To the OP, you can still compete in SchH with a dog with one testicle, two testicles, or three (still waiting to see that), just as you can compete in SchH equally well with a mutt vs a pure bred.  Enjoy training your puppy, and don't sweat it just yet.

Yvette