A dog-owning friend suggested it's a good idea to teach the dog to lay down on command. That way, when the dog starts to run for the door, you could command the dog to lay down. You like this plan because it could also stop the dog running out and getting hit by a passing vehicle.
However, you quickly learn that good ideas are all very well, but teaching a small lively dog to lay down is not as easy as it at first sounds. Not only is the dog energetic and easily distracted, but he's so physically small that he doesn't seem to naturally spend time laying down in the same way big dogs do. Teaching him is an uphill task.
Any dog over the age of 8 - 12 weeks can start to learn how to lay down. Reward-based training methods use treats to lure the pup into performing the desired action, so he voluntarily offers certain actions on command in order to earn a treat. Once the dog is regularly obeying the command, you can start phasing out the treats and use voice praise and fuss alone as motivation, interspersed with the occasional treat to keep him interested.