Dog has landed with me!

[BenT1905]

My ex bought my son a puppy with my permission specifically denied, albeit at my house.
It helped him at a difficult time to be fair, however!!
He has now left home and she, despite promises to take the dog, has not & 3 years later I am stuck with him.
He is lovely, don't get me wrong, but has massive impact on my work, holidays, general day to day life & I made it VERY clear that I would only have him for a few months to help my son thro a dark period.
What am supposed to do - still my sons best friend & he would be distraught if I were to off load him. I feel hard done by to say the least.
Everyone has got on with their lives and I am stuck with a lovely, but draining dog???? Answers on a postcard please??