I, too, offer my sympathies on the loss of your brother and understand how it can turn your world completely upside down.
As far as changing your TBs saddle and discipline, I agree with the others that it probably won't mean much to him. I used to start young TBs for the track in western, and now everyone I know starts TBs for polo in western saddles.
Several of us even use our polo "ponies" for cow work or to check pastures when they are young and during the off season. Very good for their brains and temperament.