Always give them day off the day after then big shapes the following day after having their backs done. Otherwise you will undo everything that has been done...even if just walking and trotting. May as well just give me the money!?!
You'll need to cancel your lesson. As this is the first time your horse has been checked you can probably anticipate that there will be quite alot of adjustment needed. Normally you need to give the following day of & then gently bring them back into work over the next couple of days. As this is his first time you may be advised to give a longer period.