My daughter had surgery on her Achilles tendon, but her case is different from your son's. My daughter has been diagnosed with SMA-LED. She is 4 years old. At the age of 2, she had surgery on both feet because she was diagnosed with vertical talus. Before the correct diagnosis, her feet were bandaged according to the three-plane foot therapy and stretched (for over a year). This did not improve the situation because, as it turned out, the bones were incorrectly positioned. After the diagnosis of vertical talus, we were recommended to have surgery to correct the position of the bones in her feet and shorten her Achilles tendon, because in her case it was too stretched due to the foot defect. Before the operation, our daughter had her feet in plaster casts several times for a period of about one month (each subsequent cast was in a stronger plantar flexion). The operation was successful. After the operation, she also had her feet in plaster casts for about a month. When our doctor operated on our daughter, he said that the Achilles tendon heals quickly at her age (she was 2 at the time). And indeed, she has no problems with it now. However, I do not know how recovery proceeds in older children.
PS. From my knowledge of SMA-LED, it is unusual that your son can stand and walk on his toes. My daughter has never been able to stand on her toes, but this is related to SMA-LED because her calves are so weak that she is unable to flex her feet to their full extent. Could you write more about what symptoms your son has and had in the past? We would like to learn as much as possible about this disease and its progression.

He has not had the surgery yet.