I think the idea of a family gown is lovely. And dont see the issue with a boy wearing a Christening gown - it isn't a 'dress'!

But if your DH has a real issue with it, then perhaps you should look for something else for him to wear - start a new tradition with a more masculine outfit for family Christenings for boys in the future?
I would love to know what your DH's problem is with the gown though

Good luck