A mother once came to me and said, "I want to put a christening gown on my son, but my family thinks I'm crazy!" Let me reassure this mother and all others, no, she is not crazy. It is the baptism garment par excellence for girls as well as for boys.
It is often inconceivable for fathers to dress their son, even a baby, in a gown. They absolutely want trousers, or even a jacket, in short, a little boy's outfit, and if, for better or worse, they end up being convinced by their wives to dress their son in a gown, it absolutely must be ultra-masculine so that it can be spotted at first glance that it is indeed a boy and not a girl!
When "Baby George," Kate Middleton's son and future King of England, was christened, just like all the other little princes of Europe, it was in beautiful long christening gowns full of frills and lace. Was there a single media outlet in the world that dared to criticize the christening outfit for being too feminine? No, because it is a very traditional outfit.
Now, of course, if your son is crawling, on all fours, or walking, it will certainly be more practical to opt for a romper or a little outfit. Beyond 5-6 months, a gown is not really recommended anymore.