Start by putting a small amount of massage lotion in your hand and spreading it over his back to help your hands glide over the skin. No massage lotion handy? Substitute body lotion, shea body butter, body cream, baby lotion or baby oil. Use both hands making large, slow circles to spread the lotion from shoulders to hips. This warms up the tissues as well as providing lubrication.
Use the palm of your hand to glide from his shoulders to his hips several times with increasing pressure. Note: Do not apply pressure directly to the spinal column. Now repeat this with your hand closed into a fist. Don't rush - slow, deep strokes feel better than light quick ones.
Use heels of hands or fists to dig into the muscle masses across his shoulder blades and back with as much pressure as you can. You aren't going to hurt him, and if you did he would tell you.
Where the muscles are thickest, drum lightly with the side of your open hand or loose fist. Lightly - you're tapping muscles, not chopping them up.
Finish by using the same gliding motions you used to spread the lotion.
Use similar methods on arms and legs, working toward the heart.
Alternative method: Have him lie face down on the floor, fully clothed. You sit on a chair next to him or on the floor beside him, facing him. Now use bare feet to work the muscles of his back. Your legs are stronger than your arms so you can give him better pressure. You can search on YouTube for barefoot deep pressure massage or barefoot couples massage