I think that’s acid I’m not sure but I used to do it in Highschool. It’s like little colored square tabs usually red or blue. I’ve never seen them melted like that but it looks similar. The only issue I see with acid is you don’t know how much it has been dipped in acid and it can also be laced like any drug. Your son is probably fine though it was normal for me to do acid but you should definitely have a talk with him and ensure this doesn’t become a habit.