The back problem that you describe is extremely common in many horses. Stretching is an excellent idea and would be a good thing to continue for the rest of his life...and for all horses.
As you stated, stretches to both sides and between the legs are good ones. A few tips: Don't go beyond what your horse is capable of...don't encourage "over-reaching" and thus causing strain in the stretch. Hold each stretch for a few seconds and repeat 3-5 times each. On side stretches, have him reach toward his hock and fetlock instead of up toward his hip...creates a better stretch and rounding for his back. For between the legs stretches, try not to go so far as he has to bend his front legs to perform the stretch.
Also, belly lifts are very good. Lifting the belly will raise his back. Just like with us, if we use our core muscles, it relieves our back.
A very good book/DVD is, "Activate Your Horses Core" by Narelle Stubbs and Hilary Clayton.