** update **
I just wanted to thank everyone for their suggestions. I have tried all of them and almost everything has worked! One month post-feed change, he has made a huge change. He is now on blue seal vintage performance and only 1/4 scoop of that once a day. I honestly did not know how much the BO was feeding him...only to find out that he was getting a 'heaping scoop' of straight sweetfeed TWICE a day! Yikes!
He is still Nico, a firecracker that wants to go all day. The difference that everyones suggestions has made is that he now 'wants' to go...not 'needs' to go. We went on a 5 hour trail yesterday and he was content to go my speed.
We still lunge 30 minutes before every ride which has made him even 'beefier' than he was before. When he starts getting a bit much, we circle. It started out that we were circling every 10 strides. Now, it has been a week since I have had to circle him at all.
When things were bad, I did consider trading him. I was so grateful that the trade fell through because I just love his guts. Now that we have this speed-demon under control, he is for sure in his forever home.
Thanks again to everyone who contributed! This was my first experience posting on this board and it was very helpful.