my iphone4s and his ipad 2.
This is because there are apps on his iPad that were assigned to your account. As a result, he will either have to re-download (re-purchase) them on his device (more on that below), or will have to sign into your Apple ID every time he wants to update those apps. When he synced with iTunes, it obviously synced your apps over to his iPad.

To rectify this problem, you will need to plug the iPad into iTunes and then de-select the option to sync apps (at least in iTunes 10 this should be under the "Apps" tab). It should ask you if you want to clear the apps off of the device. When prompted, accept the message. This should clear all your apps off of his device.

After this, he will need to re-download, and/or re-purchase, the apps that he wants on his iPad.