Are there a lot of saddle strings or other "danglies" on your western saddle that could be inadvertently cuing your horse to go forward or otherwise tickling or distracting him? I've ridden lesson horses that had to have saddle strings tied up to avoid upsetting them. I would definitely try swapping the fenders for English leathers and see how he reacts. Maybe he is just less comfortable not feeling your leg as clearly?
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown