HI there and thanks for the post.
I am glad you are working hard on the obedience and he is doing well most of the time. This is the most important part so well done.
I think the best way to get him to listen to you in the backyard is up to you but there are 2 major ways to do it.
1) Reprimand\
Leave his lead on him while he is in the back yard. Ask him to come to you if he sits or lies down growl at him (an AHHHH or GRRRRR works best) and tug on his lead. This will let him know that it is not okay.
If he looks or comes towards you praise him as much as you can and give him heaps of attention. If he ignores you still growl louder and tug a little bit harder.
BY praising him as he comes to you and reprimanding him when he misbehaves he will slowly learn what to do. You need to be very consistent whenever he does it you need to respond.
2) Ignore
The other option which requires even more patience by you is the ignore method. When he is not responding completely ignore him. Turn away, don't look, touch or talk to him.
The second he starts to look at you or come over praise him just like above.
You need to be very consistent.
Let me know which way you want to go and ask for help whenever you need it. You need to be very patient and i promise it will take time
Good luck
Todd