That is not my experience with OTTBs. I have had a bunch of OTTBs, and broke a lot of two year olds in race training, and I've never had be that sensitive to the stick. Hard for them to be successful racing if they are. As a matter of fact, one of the things we did when breaking babies was to desensitize them to the stick - rub it all over them, wave it around their faces, and do constant whip twirls and fanning the whip at the walk.
What's the rest of his flat work like? Is he speedier going towards the barn and slower away? Does he pop his shoulder in the corners on the flat as well?
Also, what's changed about the rest of his routine? Is he now turned out with a horse that he's now very bonded with? Is he getting the same amount of turnout?
You can get a stick, spurs or stronger rider and make him do it, but I'm not sure that will solve the underlying problem. Even if you succeed in forcing him down the line of fences; it won't be a pretty or relaxed perfomance. I would lay off the jumping for right now and vary his routine - trail rides, flat work in a paddock, add lots of lateral work to your flatwork, etc.