My Rot

Well a good option is to put your dog inside the house. I mean it may be hard to convince your mother in law, but that's a good solution. Re enforce the fence.

People will not shoot rots if they see them. They might either run or try to get it to go away. Or call animal control. Make sure your dog has a collar and the rabies tag on him at all times.

As far as rottweilers are concerned they are very obedient dogs and can be perfectly learn discipline at any age with the just one weapon or trick you have and thats Love, in the form of your care and attention toward them.
In your case a little more effort is required from your mother in law, if she can, just more intimacy and loving attention is required from her to your dog, this leads to your dog doing the best to impress her by responding to her commands and whenever he again try to go out of the fence, your mother in law just can withdraw her affection by keeping him in and let him know that this unwanted fence crossing is not at all desirable act but its punishable by keeping him in while just avoiding him.
Yes, this require little effort, but as you know how serious can it be otherwise!!
Or, Other option is..., As you can, just take the little help from animal/dog trainer, by paying some bucks will give you the best bargain for your obedient and wonderful dog and you already love him much as your query shows.

Your dog does that because he wants you. He's trying to find his pack. Your mil needs to pack with your dog. Let the dog stay inside with her so he feels welcomed and loved.