How do he tell his mom??

My boyfriend and I been together several months. We are expecting our first baby. He's told his mom about me but we haven't met yet. He's stationed in Oklahoma ( Army) and I live in Kansas. I just hit my 3rd month of pregnancy. He wants to tell his mom today ( Father's Day) but thinking of a creative way. She has arthritis in both legs and struggling right now ( single mom) we are 22. And everyone consider that too young. We are deeply in love and want to make sure we aren't hurting the ones we love by telling about the baby because of our age. Any helpful advice??