Pup wash christened in DH's christening gown - a long white one. That was a catholic ceremony. DS also got a white stole which he will use for all sacraments from now on.

A fried of mine had her baby christened at a Lutheran church and he wore a white suit.

If you don't want the long gown (usually a fmaily tradition thing), I'd just go for dressing DS in a nice white outfit. Regardless of the faith, white seems to play a part in the ceremony somewhere.