I agree with others in saying he doesn't look so Quarter Horsey...
We too have ranch bred QHs (cutters. reiners, and reined cow horses) and all but one look like the standard AQH. The one, Luke (reiner), has the same face and neck as your guy, he's a bit slimmer structured then the rest of our herd, but his body still has the typical QH muscling features (especially in the hind-end) just not as buff as the others... Your guy doesn't look to have a very Quarter Horse type body either... I'm not saying he's not, he very well could be, just not up to the "breed standards." There are other factors that can be played into his appearance such as:
- how old is he?
- how long did you have him for?/how long was he in work for?/what kind of work does he do?/how fit would you say he is?
- any lameness issues/stiffness - chiropractic work needed?
I have never heard of ranchers teaching their horse to do a running walk... it's just not a natural gait of stock horse breeds and actually, the 'western jog' is suppose to be smooth enough for riders to sit to it all day, therefore there shouldn't be a need to teach the horse a different gait.
My guesses are:
Standardbred, Fox Trotter, or QH mix with one of the others mentioned....
He's a very cute looking horse, whatever he is!
(and the filly is adorable too!
)