My mare recently had to be reconditioned... she was very sore and it took several weeks of just walking under saddle before we could add in 2mins of trotting. At first when the chiro gave me my schedule, I was a little stunned. I was ignorant. It was the best thing I could have done though. We all forget just how important the walk sets up everything. There's so much that can be done at the walk also. My mare and I are at a level now where everything is starting to click because I didn't rush with her. Not suggesting you would, but even if you spent 6weeks walking about consistently for 20mins or so, that's going to help him with getting to understand what he's like and finding his rhythm etc. It's also going to help him learn to carry a saddle and rider without the impact of a faster gait.
I think it sounds like you're on the right track. Just be careful not to overdo the lunging. Standardbreds that have been in a cart are naturally heavy on the forehand. That means circle work (even in large circles) can be impacting on their shoulders. I don't lunge much anymore, if at all, simply because my mare has been sore through the shoulder in the past. Try walking him out inhand... or long reining him... that way he won't be going around and around in circles. Especially with him being so long and having to try and pull himself together, I would be cautious not to lunge more than twice a week and for not very long at all.
Hope this helps!!