This guy may have just randomly stopped texting you because he got bored with the conversations.
Perhaps the both of you were constantly talking about the same topics and he may have felt as though the conversations had gone as far as they could go.
Being that the both of you have been texting each other non stop for three weeks, he may have simply run out of stuff to talk to you about.
If you look back on the last three weeks that you have both been texting each other and discover that he was the one who was primarily keeping the text conversations going and coming up with new topics, that may have been where you went wrong.
He may have just randomly stopped texting you because he got tired of having to be the one to keep the conversations going.