Tell him what you just told us, that you are married and happy with the life that you are living. Let him know that you are flattered but just wanna stay as friends. But just like explain it to him in a nice calm way.
I have just found out that i have an admirer. I'm married, and he is messaging me a lot, now. I haven't encouraged him at all. He is someone I know, a friend of my daughter. But how do I deal with this?
If you're happily married, then just ignore the messages or block him or come right out and tell him to please stop as you are not interested. It's easy to be nice and say you appreciate it, but you're happily married. If you're not happy with your marriage then it's a whole different story.
Tell him that you're way older than him and this is not how you approach a lady. If he doesn't stop, then tell his mom.
If it is unwanted attention, tell him you're happily married, hopefully he will respect this and leave you alone, if not block him.
Personally, and as awkward as it would be, I would have no problem being straight forward with someone who is attempting to inappropriately communicate with me. That can be done with tact and without humiliation. Just say the words "I want to say this as politely and kindly as I a can but ... Please don't text me anymore" .. If he continues, block him.
