TOTALLY agree! I know of tons of successful driving horses (Champions and more) that are cow hocked. Lots of times, this is what turns them from a Halter show horse to a career driving horse in show homes.
I'm also a fan of taking him backwards over low cavaletti's as it further strengthens the hip and leg muscles. Good diet, and it sounds like between the good farrier care and the chiropractor, he may do just fine, and turn into a good driving horse for you.
I'm also thinking he probably hurt himself, if this wasn't evident before. If he was a little cow-hocked before, and you say he came from driving lines, then this is probably his perfect inheritance and may have been a trait he inherited.
In YOUR future, if you want to show in other than driving, you may have to decide on a different stallion, to keep those perfect legs and be successful in halter, etc. But for now, look at your mares with a critical eye, and see if they have straight legs for halter, or if you may need to pursue using a different stallion for them. You can always do an "outside" breeding for a stud fee, which is certainly cheaper than purchasing another stallion for that goal.
Keep us posted! We're excited to hear how he comes along!