My boyfriend and I often use modal particles such as ah, ah, hum, 55555 in short message chat. Is he immature?

It is impossible for a third party to know whether it is mature or not. What tone you use to speak is just a personal hobby, or it is because you use these words that you will appear mentally retarded and stupid in front of you. As for asking what you saw, I should want to know more about you. It's all my personal opinion. .