I think it's pretty unanimous... change his name back to the one you wanted darl. I can think of no reason why it should be your FILs. I suspect that if you let them bully you into what they want it will be the start of a lifetime of them insisting you do things their way: should he be Christened? Should he be weaned? What school should he go to? it will never end. Nip it in the bud now.

Also I agree with 7th Heaven: if he is a June baby you are most welcome to join us in the June Babies group! Good Luck! Let us know how you go!