If it's similar to William being called Bill, which it sounds like it is, then I wouldn't worry about it. When he's old enough to do all the legal paperwork stuff, he'll know his real (legal) name.
To me, Jack is just a nickname for Johan. The same as Henry would be for Henrik.
Eta: not sure if you do public school or not, but most forms have a legal name field and then the field that asks what child would like to be called.
Sent from my Nexus S 4G using DS Forum
Trace, married to Shaun 2.21.04
Kyleigh 11.8.04 ~ Tegan 12.29.06 ~ Emmerson Kate 2.21.13
Forever missing Colton 11.5.11 (15w)
Last edited by TS8213; 09-29-2012 at 05:37 PM.