A friend bought a puppy hoping to show him ,she said she could feel his testicles at 15wks old, but at 6mths, there doesnt seem to be any, or they are high up in the abdomen, how long would you leave it to see if they do come down?

i had a GSD whos testicle didnt drop until 11 months,  i too had felt both of them when he was just 8 weeks so i decided to wait and see until he was a year old, i was lucky !
it could be you could feel both of them when he was younger but as he has grown the tube has not grown with him so one testicle has got stuck inside, your vet would have to check this out.

good luck

With a vet who is good with ultrasound scanning you should be able to find out what is going on. 

Depending on which side of the Inguinal ring they are will depend on whether they are too large to descend now.