I am having a similar issue with my Embden goose, only I did not identify it as tendon slip right away and didn't locate a vet to see him until over a week later. The vet is NOT an avian specialist, but has treated farm birds for injuries, being egg bound, and other things since we live in a small, rural farming community. She said that putting birds under anesthesia is difficult and she didn't offer much information about surgical options because I asked thinking the plan would be similar to what your vet offered you.
She did offer laser therapy... in her practice she has successfully treated a variety of animals with the laser and finds it especially effective for lameness. She is also giving him meloxicam, which is an anti-inflammatory in hopes that it will loosen the tendon enough to move it back into place. There is a thread on this page that had lots of pictures and good suggestions on braces for a gosling leg that may work better for you:
https://www.backyardchickens.com/threads/gosling-leg-problem-please-help.1536131/