Your cellphone may have a spam filter on text messages. Open the texts you have been getting from this person and select the menu or more button. If you phone has a spam option it should pop up for you.
This blocks messages from that source. It won't prevent him from finding a new source if he figures it out.
Another option is to ask your carrier for a new phone number. Just make sure the perpetrator of the harassment is not given the new number.
Sounds like you're doing all the right things, good luck.