Thank you both very much for your prompt answers. I had just looked up knobber in Chambers and saw that it was the same as brocken. Why do they need two words for the same thing; it's not as though they are words from different regions of the country.
That is very strange, because if you look up knobber on the same website that Sarumite has posted it also says a 2 year old stag.
Several other websites say that knobber is a two year old stag, and a brock a 3 year old stag.
Forgive me if I seem to be nit-picking, but FoxLee asked for the name for a stag "in its second year" not a 2 year old stag. Would this not then be a yearling?
I thought this was a relatively straightforward question! You've made a good point Dustybun, I shall probably go with your yearling. Many thanks and to everyone else for their input.