Hi, there are lots of ways to deal with this. It sounds like this horse doesn't have too much respect, so you could start by doing some ground work exercises. Teach him to back up and yield his hindquarters and fore quarters (let me know if you need help learning those exercises) in a place where you're comfortable, and then lead him out on trail. If he stops, make him work more by backing him up and making him yield and move his feet until he does what you ask. Make it his idea by making him work a lot when he doesn't move forward and letting him rest when he takes some steps forward. Try to change your mind set so that you communicate to him, "I don't care what you do. You can choose to stop or go, but when you stop, you're just going to work a lot harder. It's your decision." It might take him a couple of minutes to understand but if you're consistent and don't give up, eventually he will. Then, you can transfer that idea under saddle by teaching him to yield his hindquarters under saddle (once again let me know if you need help teaching him that), which is much more work than simple walking forward. Teach him that ahead of time as well. THen, whenever he stops, make him yield over and over again in a lot of tiny circles, and then suggest that he walk forward - I guarantee he'll choose to walk forward. This way, you won't fight with him as much and itll be his idea to keep going.
Let me know if this works! Or if you have any other questions ! :)