I know what's going to happen.
if you start reception as bear you'll end up staying iwth it unless your ds makes a concerted effort to change it.
by the time he goes to senior school it will be entrenched and people will know him as bear. like all his friends and the teachers who know him.
and then you'll get the substitute teacher who doesn't know his name and is using the register. even if they put bear in and underline it they'll miss it.
they'll then shout xx name to him and he'll be looking out of the window wondering whether it's snowing yet and he can get sent home or whatever crap runs through the average 11 year old's head.
the whole class turns towards him and shouts "bear man" he means you and then Bear feels like a tit and got caught for staring out of windows.
this then carries on through university, through dealings with police (real name on driving licence). it's embarrassing in court if people talk to you and you don't respond. and everything whilst your parents laugh at you for having a problem with it.
he then would never in a month of sundays do it to his own children cos it's been a right pain in the arse.
if you sniff bitterness you're right. only difference for me was that both names are what people consider real names raher than nicknames. i worked for someone who had gone through the whole thing with a nickname. that was a nightmare for her but she didn't reply to susan because she didn't hear it - most people thought that that was an affectation. it really wasn't.