Just as important as commands such as "sit" or "stay" are the command to release the dog from them.
You want to be able to control what your dog does, and to do that it needs to know when you say "stay" you want it to stay until released.
If you don't teach your dog a release command, then you are telling your dog it has to stay but only for as long as it wants, rather than as long as you want.
You can choose whatever you like as your release word as long as you are consistent.
As with all commands, try to keep it short and concise so that there can be no confusion.
Common commands used for release are "free" and "done".
If you can associate a simple hand gesture with the command then that would be even better.
To start to get your dog used to what the release command means, you will need to get him to move around when you say it.
You can do this by distracting it by making noises, or moving yourself so that it follows you.
Make sure you issue the command only once when the dog starts to move.
It is often not necessary to use a treat in this situation.
The release command is often overlooked by new dog trainers, but in reality it is just as important as any other command.
It is one of the easier commands to train as your dog will always want to move from its position without being commanded, so it is just associating a command with that desire.