Keep a pocketful of treats and as previously mentioned, reward the positive. Walk quietly around the arena, stop him and offer a treat. Repeat only taking a few more steps, treat. I make a cluck sound with my tongue preceeding the reward so he knows it's coming. Horses are motivated by food and often learn 10 times faster if rewarded with food. This often has farther reaching positives that just what you are training. Be sure to tell him whoa each time just before you stop him, click and treat. Whoa will begin to take on a positive for him and if you sense him getting excited he's more likely to respond to it. Slow and steady wins. Don't worry about doing anything else with him, just get him solid at the walk. You can change it up with walkover poles, etc. Try this, have his front legs step over a pole and see if he will stop while straddling the pole. Horses want to keep going but it will help you reinforce his whoa while adding a little interest.I hope you are using a mounting block. When you mount, click, treat, then dismount. Repeat until your nerves settle down.