Circles circles circles! Inside leg on pushing him out and steadying him in your outside rein. Getting him to relax over his back and lateral exercises help too! Help him feel confident in himself and letting him know you're going to be there for him
Inside leg to outside rein. Keep your outside rein steady and connected, and aid them to soften up and bend with your inside rein. LOTS of circles help a lot too
Make sure your horse is going forward, from your inside leg to outside hand. A horse cannot bend properly unless it is working correctly. Keep a steady outside contact and then ask it to soften with the inside rein. Shallow loops are good, don't try tight circles, keep everything forward and relaxed. Transitions help too to get their back end underneath them which helps bending :)
A really good exercise is to make Some S-figures but don't try to overbend the horse to much. Good luck!