Sit: If he will jump for a treat, he will sit too. Instead of trying to put the butt down, stand in front of him and hold the treat lower than for the jump and more or less onjust above the snout. The head will come up and the butt automatically goes down. It may only be a partial the first time, but praise & treat him for trying. Try again making him get the butt down farther.
Another suggestion for sitting would be to get a wall behind you and hold a treat above your pigs head, not to high as you want him to reach up so that his rump goes down. If you hold it to high he will jump up for the treat. The wall behind him will help so that he doesn't back up and his rump will be forced to go down. If he succeeds say sit and treat him. Eventually he will learn to associate the word sit with the trick. If he doesn't do well with this, don't worry, not all pigs like to sit. I think it has something to do with their shapes, some like to sit, some don't.
Wave: Get them to sit first. Then, while holding the treat in one hand, say "Wave" while using your other hand to give them a signal. I started with an upside down wave, briefly, then taking that hand and reaching down and tapping the back of her front leg. When he lifts it just a little, give him the treat and praise him. Eventually, you can change your signal to look like you are waving at him.
It helps if you have a greedy pig. Repeat four or five times in the morning and then the same at night. It doesn't take long. Mine is always waving at me trying to get me to give him a treat. And it's hard to resist a waving pig.
Circle: Try teaching him to circle (or spin as some call it). I found this to be the easiest trick to teach. Get him to follow a treat until he completes a circle and say the word circle or spin and give him the treat. Eventually try to get him to do a circle just by saying the word or using a hand signal. Once he gets this down increase the amount of circles or spins he needs to do before you give him the treat (without making him dizzy, that is).
Have fun and be creative.