Vicki in Ky

We just received an 18 month old male mini. He has been living outside. He is not house trained. We plan on training him to be a house dog. He has not been neutered. He keeps marking his terriority all over the inside of the house with little "sprays" of urine. Is this because he has not been neutered? Will he quit when we get him fixed? Is he to old to have him neutered?