Touchscreen acting strange after screen repair

My 13 year old son cracked his screen and didn't have the money to replace it, so he decided to fix it himself. He ordered the screen and tools and watched a lot of tutorials. He got the new screen in place, home button and camera installed, and everything seems to work perfectly, until he tried to type something. Whenever he tries to text or type a comment, the screen gets erratic, scrolls up and down and types the wrong letters. The backspace key is the worst problem though. When you try to backspace, it acts like a return key and/or types the letter M. The touch response seems fine in every other way. We've double and triple checked the 3 main connector cables, and they seem to be in the right place. Any idea what could be causing this???