FIRST thing first....
look up your state LL tenant laws and read them.
Most states ( if any) do not recognize e-mail and texting as legal notice. Yes we do use it for easy communication at times, but if there is any "legal" issues that may arise ( of it is from a problem tenant) we insist on those communications that are recognized. Even though phone calls are recognized, they do not hold up well in court so don't depend on them.
Some states ( I understand) require or allow certified mail --- others forbid it (my forbids it). If not required to be certified, send it first class and get a "certificate of mailing" at the post office when you mail it. It costs less than $1. Attach that to your copy of the letter that you keep. It will be proof positive that you sent it and hold up in court.
Now, you goofed up big time. Did your contract say the deposit was paid? (you do have a written contract don't you?) Hopefully you made note that it was due and not paid. Why 2 checks??????? just get the money and move the deposit to the separate account!!!
Best advice is to get the notice per state law and try to come to an agreement-- maybe he will be a really good tenant and is just short of money ( hopefully you did check out references). Get him to send in payments if nothing else. If your state law allows ( most do) personal delivery you could have you mother do it ( have it signed for) or post it if allowed ( usually requires mailing as well) Be firm, be fair, and follow the law. DON'T have your mother discuss anything -- it could bind you to it or you could be held liable for anything she says. She can -- deliver the notice and pick up a check-- nothing else.