what does it mean if a guy keeps texting you when you don't text him back?

This friend that's a guy will not stop texting me constantly. If I don't text him back within the next 5 minutes he will send another text. he has told me that he likes me and would like to go, but i told him i just want to be friends. but really come, who does this? This seems different from a person who does like you texting you and maybe a crazy person texting you.

One time i forgot to text him back, then when I checked my phone he sent me around 20 text messages

most of them are like... "what are you doing" "what's up" "what are you doing, text me" "Hey what you doing" "what you doing right now" "sarah?" "Are you there?" what you doing, huh?" "Are you near your phone?" Sarah?" "how is your day" "What are you doing"....this is just one of many group of texts he has sent me in this order.

it's the most annoying thing ever. I've even told him about it and he keeps doing it. What does all this mean? sometimes i don't even want to talk and he'll just keep sending them.... oh great now he's calling me right as i'm typing this. Sigh, help me.