While you may never enter your pooch in an obedience contest, there is no reason why he can't learn all of the same commands used in these competitions. One thing most people tend to overlook is that their dog is far smarter than they realize and is capable of learning just about anything as long as you are willing to put in the time and effort.
The more you can make this particular training into a game, the faster your pup is likely to learn to drop in his tracks when you give him the command. You should teach him to respond to both verbal and hand signal commands so that he will follow your command from a distance as well. Teaching him this command may just save his life at some point in the future. Drop on recall is not that much different than teaching your dog to drop or lie down when he is near you. The big difference is that he is in motion and must stop first and then drop to the ground.
The definition of this command is to teach your dog to drop in place when he is coming towards you after you have called him to 'come'. The command words are up to you, as are the hand signals. Most trainers recommend a simple "Drop!" command and extending your arm out palm down, raising it up so that your dog can see it and then dropping it, still extended palm down, towards your dog. Before you can teach him to drop on recall, you must first have taught your dog how to drop on command, as this is a natural extension of that particular command. In fact, you should be using the exact same command for both.
The command should not be difficult in and of itself as your pup should already know what is expected of him when you say "down." However, since your dog will be in motion (quite possibly running towards you at full tilt), getting him to slam on the brakes and drop on the spot can take some time so be patient and work with him until he gets it down. While you can teach this command to your dog at any age, he must first have learned 'come' and 'down' before he is ready to move onto learning this skill. Just remember patience is its own reward and teaching your dog 'down on recall' could save him from serious injury or worse.
The first thing needed when trying to train your dog 'down on recall' is for him to already know commands such as 'stay', 'come', and 'down'. You should also have a healthy supply of his favorite treats, tons of patience and a quiet distraction-free area to start training him in. Avoid areas where there are distractions such as other dogs, kids playing, traffic, and anything else that might break his concentration. As always, when giving your pup a command, your voice needs to be authoritative and firm, letting your dog know you mean business without sounding like he has done something wrong. With a plenty of practice and patience--not to mention lots of treats and praise--your dog will soon learn this new "trick".