A serpentine is almost like an S shape, but with more loops. So, like two S's put together, or three, depending on the size of your arena.
Are you finding it difficult to work him because he is hyper? I know of a horse that was like that. And what his trainer did was just let him lope first. Let him blow off steam, stretch his legs, and work off that excess energy.
Once you feel him tire a little and start to slow down, then go to work on training.
Another exercise that teaches a horse to slow down is straight line turns. That's where you start at a walk, walk in a straight line, then turn (45 degree TOH). Walk a few steps or many steps, and turn again. Change directions, and change up how far you walk before turning. That will teach your horse to slow and wait for you to ask. Once he does it well at the walk, push it up to a walk.