There are alot of ways. I use a variety on my boyfriend who is also my sub, Start with making him wear panties all the time. Second, spanking always works. Not torture but reasonable spanking. Use a strap or a belt. Have him stand in front of you and explain that he has to be spanked as part of his training. Then tell him that from then on when you tell him he is to be spanked that he is to report to you and he is to bring whatever instrument you order him to.
Then explain that he must then remove his jeans or slacks. Depending on whether you decide it will be over the knee or not will be up to you. If so, tell him he is to pull down his panties and get over your knee. If its easier for you, tell him to pull his panties down and then bend over a chair. Tell him he must count the strokes and then thank you after. Give him 15 to 25 strokes with the strap or belt and when its done rub some oil on his behind to cool it. Have him stand in a corner for a bit then tell him he can put his panties on. Another item that is a must is a collar and a leash.
Make him wear a collar all the time and train him to put it on himself when you go out. A leash of 6 to 8 feet works but also have a short leash of 4 feet on hand. Finally, and this is very important, put him in chastity. Mine wears a steel cock cage. He is allowed out of it to cum at least once a week. On those occasions he always is to cum in front of me. A couple times my girlfriends wanted to watch. If he doesn't wear it all the time, as mine does not, train him so that when you order him to put the cock cage on he does so in front of you, He must lock it and give you the key.
I also use restraints of various kinds. Leather wrist cuffs and ankle cuffs which he must put on and then which are locked. He has to attach the chain to the ankle cuffs, lock it and give me the key. I also have a full set of standard transport restraints which consists of shackles, a waist it chain and handcuffs.
A couple things that may happen. Mine often gets a vary hard erection when he is about to be spanked. That's if I take the cock cage off. I noticed the first time I put him in the transport restraints he got very hard. All of that means he is becoming well trained.
One final item is diapers and plastic panties. Get tight plastic panties and put him in diapers and the panties now and then for the night and when you go out. He can wear the cock cage under the diaper. You might also want to get locking plastic panties. The ones I use have a small chain at the waist which is tightened and then locked with a small padlock. I also ave a thin waist chain that fits over the waist and has another chain that goes between his legs. Even if he is in restraints this reminds him of who is in charge. Good luck