Don't down yourself mate, it's not your fault.
When he locks right up, you can push the ligaments back over the patella by pushing against the stifle with the palm of your hand in a circular motion. Get your vet to show you if possible.
Generally there is no pain when the ligaments slide off the patella, rather discomfort and some stress if the horse can't walk. If your boy is getting that bad, I would be talking to another vet for a second opinion. There are a few options available, surgery is one if you want to go down that path. The ligaments can be cut which frees the joint up, but can cause bone chips and some arthritis down the track.
Another option is to inject cortisone and a lubricating agent into the stifle to allow the ligaments to run more freely over the patella.
It is actually better to have him out in the paddock, if it is true stifle lock, keeping him moving will help to free the joint up. Paddocking him on a slope would be ideal. Much like arthritis, the more he moves around, the more lubricated the joint will become.