Basically, you can use whatever you want.
Any signal you want to use can be paired to a behavior. If there's an existing signal (including verbal), you can teach a hand signal by giving the hand signal first and saying your verbal at the same time.
For example, when I taught Wally that my palm facing down and moving down to the ground equaled "drop" (lie down), I gave the hand signal, then the verbal "drop" and rewarded when he did the behavior.
You can also try luring to develop some hand signals. To use the previous example, I could have lured Wally to lie down by moving a hand with a treat in it to the floor and releasing it only after he lies down. Then I "fade the treat" (take it out of the picture) and just do the hand signal. The treat then comes after he lies down.
So there's multiple ways to approach it and any hand/body signal you can reliably repeat is fair game. Just make sure you can do it very close to the same way every time, especially in the beginning as he's just starting to learn it.