Most young people do not use cheques at all. So, he has probably put it away somewhere 'safe' until he gets the opportunity, when he remembers, totake it into a bank. I have g.children who get really quite putout by a g.parent who insists on sending them cheques, and it really is quite an inconvenience for them. So, often they actually cannot rmmeber whre they put the cheque when they are going somewhere near to a bank. In future, find out their bank account details and do a bank transfer with the money. Safer for it, and ensures it reaches them.
Yes, obviously would be pleasant if they got round to saying Thank You. Perhaps if they had your email address then they would be more likely so to do. How, at present, do you expect them to send this? However, in my mind not the most important thing I give cash to all my g.children of secondary school age and above. TBH- I do not expect a Thank You from them, just pleased that I am able to do this. My AC receive money from Xmas directly into their Bank accounts. Again, I am happy coz I know it arrives their safely, I do like it if they use it for something special that they then send me a photo of that.
Would have to search to find my cheque book.