Sorry at least I made one person laugh !!!
So you want collection

on a otttb.
Well firstly you need an instructor even if it is once a month they will be your main sorce of help.
Hopefully you would have made sure he is sound. ie teeth done, feet done, no sore spots ( my throughbreds get bowen every 8 weeks)
So collection takes many months,years even to achieve.
I am not going to tell you how to achieve it but things that have helped me. I do not think anyone can help you unless they have seen your horse and sorced the issue. That is where an instructor can help you most of all.
Firstly I would think about a running marting gale ... (i'm not going to go into detail).
Secondly I would lunge him with a shabong?? or a pessoa they encourage long and low movement that helps with the development of the back muscles
Thirdly lots and lots of circles
and do searches on collection etc on google to help you some more.
Good luck with it all !!!