I have a 5yr old dog who I intended to use as a stud dog but unfortunately for some reason he never quite got there !!!
So I gave up .    For the past week or two he will not leave my other dog who is 10 alone, he constantly follows him around,licks him,gets really excited to the point that he is ready to mate him . Please can anyone tell me why he has suddenly started to do this when previously he has not really shown any interest in mating, apart from following the bitch around
Any replies would be gratefully appreciated
Thanks

I would have them both checked out at the vets.  Sometimes, when a dog is ill or has an infection it can lead to this sort of behaviour.

Is the older dog neutered?

yes he was neutered at 6 mths old

It'd be worth getting his anal glands checked; the smell of infection can be very similar (to a dog!) to the smell of a bitch in season.

I agree have him checked over to make sure that he is well but neutered dogs often start to smell to other dogs like a bitch in heat and if that is what has happened in your case you have two options, give the old boy HRT (expensive but works after a couple of months) or have the younger one given one of the chemical castration to see if that helps (also takes a while to work)