I have a german Shepherd who is 10 this month. He is in very good health. He is a large boned Shepherd and he weighs 57 kg. The vet I took him to was not very good at all (I took him for his booster injection).

The vet said he needs to loose weight and get him down to 45 kg and then as one of his testicle seems rather hard she wants to castrate him as it could lead to tumours? Can anyone advise me if they think this is normal in a healthy dog? I have jost lost a German Shepherd earlier this year with hip D and I was shocked as she said the surgury is not without risks at his age.

If a dog of 10 has a change to his testicles, it is better to castrate than risk a tumour spreading a killing him.
We just faced the same quandry with our 11 year old Champion male. We decided to go ahead with it, and it has made absolutely NO difference to him and the operation did not affect him at all.

Surgery is not without risk at any age, but it's safer to undergo it when relatively healthy, than in an emergency situation.

Snomaes