If anyone has an English Mastiff talk to me about....?

Training, Mine will sit and stay for a long time if I ask him, I can open his gate tell him to stay and he will until I tell him to come. My problem is when he is out running in the yard he wont come or stay when I call him its like out of sight out of mind or something. I will NOT us a shock collar on him that is out of the question... So how can i train him to come when he is out sniffing about and ignoring me. I dont want him to run off and get lost thats my greates fear. Please help?