Slowly increase the horses work load as directed by Julia Bowen. Make sure that the horse is rounding out over his back and lifting his stomach up beneath you. For more information I highly recommend reading basic articles on dressage. It is a little to complicated to explain here. There are also several in hand exercises that can help including lounging with side reins and long reining, but I recommend having a professional teach you these excersices. With the consent of a professional, draw reins can be used to encourage the horse to stretch over his back and lift his stomach, but this can be harmful for the horse if used incorrectly. I hope this helps!
YES!!! Start with just walking, if you can "trail" ride even just around your property, that will help. I do LOTS of walk, walk/trot transitions, poles. My gal has been off most of the winter so I've been starting with just 15 min. then I'll go up to 20 in about a week then add 10 min after a month.
You can do interval training but start slowly so he doesn't overwork