Oh Dear, poor you, I've been in EXACTLY this situation with my own Mum, so feel for you & YADNBU
Mine has been trying to get DD one since she was 3, along with the adult keyboard, drum set etc that are still boxed in the loft & meant us replacing with kids versions 
Theres no reason a 5 yr old needs a laptop, it also means when they ARE old enough to get the benefit from one, the seeds are sown for it to be outside of your control, in our case I have NO intention of allowing DD her own until she's an adult, that way we have every right, without arguments to control what she does on it.
In my Mums case it was on the surface a kind, but in truth a very controlling gesture, so took a lot to stop of arguments to stop her.....more recent years, she buys what she likes
however in appropriate
& we have to replace it with a more suitable alternative to avoid WW3
, luckily we don't live near by & she won't visit us, so we can get away with swapping adult for toy, though as the years have gone on, thats got harder as DD knows whats what, so its just stress
....she now buys DD PC games...even though we don't own a PC, because she wants to get DD one 
IF its not a control thing, then the answer is simple......let her get him one of the V.smile toy ones or something similat, & put the difference into his trust fund :)
IF it might be about control, you need to put your foot down & get yourself a flak jacket...good luck :)