Our parents purposely gave us names that couldn't be shortened for that reason. DD2 has had the shortened version of her name since she was 4 months old, even though I didn't like it (I started it ) yet DD1 was always her full name. Because if I shortened it ex hated it... yet part of the reason I liked that name, was because I liked the shortened version...
We did the same with ours, giving them a name that couldn't be shortened. People now deem it appropriate to add a "y" to the end of the name. People will butcher anything if given half a chance :P

Ive had DS come to me and ask me to approach his teacher to get the cutesy name to stop. He had already asked and they kept doing it, so he sought out my assistance to back him up as it were. I spoke to the teacher discretely and it stopped. In this case it doesn't sound like your DP backing him up though.

I hope you can work something out with DS so you can meet at some happy medium.

FWIW my mum still does the same with me and I do the same with DS in regards to always being my baby. DS hasn't said anything... yet.