Teaching a stay cue.
Use a mat to begin with. I prefer a raised surface (I use a platform) as it really helps to define the space your dog should be in. Start close and reward him for being on the mat. Give him a treat every few seconds for about 5 or 6 treats. Then take a small step backwards and quickly return to him. If he hasn’t moved, then you can give the treat.
It is important to return to your dog for rewards (initially) as it is the ‘not moving’ part that needs reinforcing. Pay the Stay!
Gradually increase the distance but always return to him for the reward which reinforces the ‘not moving’ behaviour. Once he shows that he is beginning to understand the behaviour you can try adding a visual or verbal cue. Make sure this visual cue doesn’t look like anything else. I use an open hand with palm facing the dog. Keep the sessions short (1-2 minutes max) and then give your dog a play break. Do use a release word (like “OK” or “FREE”) so that he knows he has permission to move from the stay position.