
Are you sure of the diagnosis, could he have said " hyperextension " of the hock [ the joint extends farther than it should]?
At 4yrs old I would have expected it to have shown up by now if he has had this since birth, he may have done something energetic which has led to the sudden onset of the pain, it may settle with rest and the pain meds, if there is no improvement then ask for a second opinion/referral to a speciallist.
If you apply for insurance for him now this will be classed as a pre existing condition and therefore not covered by the insurance.