This is an interesting issue, but a common one I come accross with some clients. Lots of times my clients will want to switch the event they are riding their horses in. The horses must adjust to each new style of riding, and the owners expect perfection in each.
Understanding that you have a TB, and a former racehorse, are the primary reasons you see behavioral changes in your horse between riding western and english. Your horse is now given a spice of his former track days when he is allowed to move forward when ridden english.
There are some TB's that do well with western riding if they stick with it. Some are even good at frequent conversion between being hunter/jumpers, and western horses. But it takes repetition and lots of disclipline and training technique to get a horse who would rather be riding english to stay focused with western.
First ask yourself if you really need to work your TB western. Is it because you like a western saddle? The slow and casual western pleasure feel when you ride? Maybe you like goig on trails? First decide if you western is even an option to continue with. It is possible that when your horse was ridden western, he may have lost an interest, became bored, and hence was being a great western horse for the time. It is possible that he was given the chance to relax and go slow and simply enjoyed it and did/does well at it.
From what I can tell, your horse may have great potential as a jumper. Not only because he is a TB, but because he has clearly shown an interest in being ridden english again by getting pumped up. He is getting excited again, and his willingness is showing. This is a great time to introduce him to the basics of jumping and see what potential he has for this.
Jumping horses are just as much athletes as pure racing horses are. Horses that jump must have the stamina race horses do, but they must also have the ability to jump huge obstacles as well as ride well under saddle and in control of their ground work.
If you are determined to keep him both english and western, there are techniques to help with that. For western riding, he may need some light lungeing bitted up with a form of a tie down, chambon, draw reins, side reins, etc, attached to the saddle while being worked in the round pen or on a line. This will remind him that he is to stay in a tighter western frame, perorm slower and even more rounded and collected, and to stay off the bit as much as possible. Lost of transitions will also help. For example; a walk to a (jog/western) to a stop and then a backup. Those are normal basic wp moves. On the other hand riding him english you would use transitions going up such as the walk to the trot to the extended trot to the canter, then to the trot again. See the difference?
Looks like i'm writing a book here, lmao....well I hope this helps for now, let me know how things progress :)