There are several synonyms that can be used in place of the phrase "big boy." For example, a grown man can be referred to as a "mature gentleman" or "adult male." Another informal phrase that could be used is "big guy" or "tall fellow." Additionally, more colloquial terms like "dude" or "bro" can also be substituted for "big boy" in certain contexts. It's important to consider the tone and formality of the situation when choosing the appropriate synonym for "big boy," as some options may be more appropriate than others depending on the context.