The notes about the front foot are right on. Also, he should square up to the wave more. One tip we do with a lot of new riders is use a rope or adjust a rope so that its length is set so that when you are in the pocket, there is some slack but not too much. Tell the rider to NOT choke up on the rope, but slack it. This will force them to lean forward and learn to catch up to the boat instead of pulling on the rope. At that point, have them let go of the rope with their back hand and only hold with the front hand. This will help them square their body better to the wave and get the right position. Tell them to make small corrections, not big ones and tell them to take their time and get stable. Once they can stay stable in the pocket with slack, instruct them how to let go of the rope. Most new riders will want to throw in the rope like everyone else does. Once you see them stable and slacked tell them to let go when you tell them to, but only to LET GO, just drop the rope without throwing or moving their hands. You, in the boat tell them to LET GO and then when they do snap the rope away quick so it doesn't splash or tangle in the board and make them fall.
Once they let go tell them to hold their arms out toward the wake, like superman. This helps them keep their weight on their toes and over the board. These are just tips, use what works. I never tell a rider all of this at once, tell them as they learn. First tell them how to get up, then tell them how to position their feet, then tell them how to make small adjustments, etc. If you tell them all at once its usually too much.
Have fun and post pictures when they get up. You may find some good tips in this video we did some time ago, hope it helps.