I knew someone who had a shetland that had a worn hind hoof, i think he had a locking stifle, aparently common in smaller breeds, she used to back him up to relieve it every now and again.
Lots of postive thoughts for little cob - if you are genuine enough to love your animals it is so completely horrible to see anything you care for not 100%
But in my (all to plentiful) experience time really is the greatest healer - and if you are only 3 years old it should be something in plentiful supply. So if I was you would just mentally write off the summer as far as any serious work and just have growing up time - and if it changes then that is a bonus!?
Also, agree, the weather is so unpredictable at the mo that who knows what is happening to the sugar levels, in the grass, so if you can be bothered to keep horse in at least part of the day it would make sense!!?