... As for all those years you didn't gift him, doesn't matter an iota. It wasn't the OP's fault either for not being able to gift him all those years and the very first oppurtunity to do so, she did. And he couldn't even thank the OP verbally. No matter how someone was raised, please and thank you is very basic skills that everyone should know.
You do know we're not talking about "gifts" here as in the 'wrapped up presents' sense of the word, right?
We're talking about this teenager (a tough enough time of life anyway) who has never had the 'gift' of family here. No, not his fault. No, not the OP's fault. Circumstances beyond either of their control determined that.
But the younger brother has had the joy and comfort of knowing this family, knowing they love him, knowing they care about him for his whole life. Will has had none of that. The gift of being part of this family has never been his until very recently.
Again not the OP's fault either. But I didn't realized there was a whole backstory of him not having family around. I thought he was with his mother's side only till recently and now able to meet his dad's side of the famly.
While I think the thank you note might be a bit beyond a typical teenager perhaps not raised that way (
I think it's a great idea to see if bro can influence him to do so since his other son does
) I do think at 16, he is accountable for saying thank you verbally.
ETA to add, just read the updates. I think movie tickets are the way to go as well if you rather not gift the same amount in cash.