Has the OP replied? I haven't seen one.
Its upsetting to have complete strangers call your GC a spoilt brat and not helpful so maybe she's blocked this post?
Sparkling, you obviously want to give the boy a cheque, but in your shoes I'd give about half the amount I gave for his 18th, and on his card I'd say now he's fully 'of age' that's the lot as far as gifts go.
If you want to, and as he's your only grand child, give him a tiny joke gift for birthday and Christmas from now on - like a wibbly wobbly glass, or a box of 'nothing' (the internet has lots of 'silly' gift ideas, just search 'silly gifts'. Things like a CD (find out his taste in music), a bottle, some mens cosmetics - about a fiver's worth, would be fine, but and STICK TO IT.
Eventually he'll get married and there'll be a wife to buy for (but only give her a fiver's worth of gift too) then the babies will follow so you can indulge your penchant for gift buying for the children, you obviously a generous person who likes giving, but please don't continue with giving money gifts to a grown man.
I hope you're not upset.