This sounds quite difficult, especially as he is your only Grandchild and you do stress his "expectation", so I think you might struggle because of this. And if you are going to gift him money - I wonder what HE is expecting? It's very hard to give a gift to someone if you are going to have to worry what is their acceptable figure for a 21st. So much depends on your own view and circumstances, because it's hardly a bill you are paying! I honestly think that it would be so much easier to consider a present rather than money, especially if he had a big bash for his 18th. Can you not speak to his parents and find out if there are any things that he would love to have as a gift, then you can consider your budget, make the choice and surprise him? Otherwise, and if you find out from his parents that money is your only option, it's probably best if you decide on a figure for yourself - I'm sure you'll be very generous, you're his Gran - and hope that all your kindness is really appreciated.