Discuss it with your son first since it a holiday for both of you.How would you compare to Koh Lipe to Surin?
I really think I should limit to one of those.
For Surin, I think I will stay 1-2 nights on the island, either in a tent or Bungalow.
Koh Chang is only few hrs from Bangkok by bus. Snorkelling is available as well as s scuba diving(not very good). BTW, there are several islands , large or small, near Kho Chang eg. Koh Mak and Koh Kood.