In Maryland, the common law crimes of "assault" and "battery" have been consolidated by statute into two substantive crimes: first and second degree assault. See, Maryland Criminal Law Article Section 3-201 through 3-203.
Second degree assault includes (1) battery; (2) attempted battery; and (3) intentionally placing another in apprehension of receiving an immediate battery. See, Hill v. State, 134 Md.App. 327 (2000).
If your boyfriend is charged, he is facing a misdemeanor with the maximum penalty of ten (10) years in jail. It is a charge that must be taken seriously. He should consult with a private attorney, or apply for representation through the public defender if he is charged with a crime.
Adding to Mr. Valkenet's excellent answer, because your ex did not immediately call the police (and because there was no actual physical contact and no injury), the only way criminal charges could go forward is if your ex goes down to the District Court Commissioner and swears out a statement of charges of what happened, and the Commissioner determines that the factual statement supports the grounds for an assault. I assume this happened in Montgomery County? If the Commissioner agrees (it is a low threshold), then a summons is issued to your boyfriend and the matter is additionally sent to the Montgomery County State's Attorney's Office for review to determine whether the charges will be pursued or dropped. It all may end there. If the case goes forward, your boyfriend should hire an attorney.
You could consider a protective order for your boyfriend, yourself, and your kids. Here's more information http://www.courts.state.md.us/courtforms/joint/ccdcdvpo001br.pdf
This is not legal advice nor does it create an attorney-client relationship. This is for education and informational purposes only. It is always recommended that you contact an attorney with any concerns as each individual case is unique.