How do I stop my son's ipod touch from receiving the texts that I'm receiving on my iphone 4s?

You are sharing an iCloud account with him.

Disable your number in iMessage:
Settings > Messages > Send & Receive > make sure your number is unchecked in both sections

You may want to do the same for FaceTime:
Settings > FaceTime

Ideally you should not be sharing an Apple ID for iCloud, iMessage or FaceTime.

He could also have access to your contacts, calendars, notes, remiders, mail, and possible your Photo STream, if you have them enabled.

You can share an ID for purchases but I do not recommend sharing one for anything else.

You can remove your iCloud account from his device here:
Settings > iCloud > disable Find My iPhone (if it is enabled) > Delete Account.

MacMost Now 653: Setting Up Multiple iOS Devices For Messages and FaceTime

You can also

Create a NEW account/ID for her using these instructions. Make sure you follow the instructions. Many do not and if you do not you will not get the None option. You must use an email address that you have not used with Apple before. Make sure you specify a birthdate that results in being at least 13 years old

Creating an iTunes Store, App Store, iBookstore, and Mac App Store account without a credit card

More details of how:

http://ipadhelp.com/ipad-help-tips-tricks/how-to-get-free-apps-from-the-app-stor e-without-a-credit-card/

Use the new ID on her iPod but only for:

Settings>Messages>Send and Receive

Settings>FaceTime

Settings>GameCenter

and Settings>iCloud if you want her to have separate Contacts Calendar and some other things.

Continue to use the same/common Apple ID for Settings>iTunes and App stores so you can share purchases.