My 12 month old neutered male dog has started peeing on my 5 year old neutered male. The older dog is a very laid back, gentle golden retriever and is incredibly patient with the youngest member of the family. He doesn't react when the little one pees on him! What can I do to stop this behaviour?

You have to replace the young one as alpha dog, by being extra firm and disciplined for a while. No coddling; lots of commands. Pretend this is the army. If you are female, you can still be alpha male to a dog. Set aside 20 minutes a day for training out in the yard, the usual sit stay come heel down, with just a pat and 'good dog' at the end of all of it. You can snuggle and pat and sweet talk him once in a while of course, such as before sleep.
Eventually he would stop anyway, but you can stop it.