Obviously once you can get out, just walking until your baby is relaxed and swinging is a good way to go.
In an arena, I really value what Alex Gerding taught me. You are so sensitive and aware of your horse it will be natural for you, and seems so obvious.
Use lateral work/bend to help get the relaxation you’re looking for. Just play with it and see what makes him looser. If it makes him feel looser, go with it. And do his easy side first, because it loosens up the tighter muscles through getting them moving, and when you work on the harder side it will be easier.
We’ve progressed to the stage of finding tight areas and working on them specifically stage, but starting by going with what’s easier is a good first start.