@yasiraffan: What a tremendous help you must be to your dad as he's going through this. It's very discouraging and disappointing to learn of progression.

There ought to be additional options that can be considered: perhaps Lenvatinib + Pembrolizumab, chemotherapy agents, clinical trials and/or others but truly it would be up to the managing oncology team who is most familiar with your father's medical history and responses to date to formulate the best treatment plan for him. Every individual is different so what works for one, may not be appropriate for another.

You may be able to find out more through https://www.curemelanoma.org/

Hope this helps and that your father moves on successfully to his next round of intervention.