Disk is good, but why won't the system boot from it?

This is my son's mac. He dropped it and told me it worked fine right after. He powered it off and the next day it wouldn't boot. He gave it to me and I tried the install DVD which doesn't see the hard disk drive. I removed the drive, plugged it into a SATA-to-USB and plugged that USB into my mac. I can see the drive just fine. I put it back in his mac, reseated the system board end of the cable, but the system wouldn't boot from it. I put a new, spare drive in his mac. I booted from the install DVD, which didn't see the new disk. I plugged in his drive via SATA-to-USB, selected it as the startup disk and did a restart. It booted fine from his old drive. I am guessing the only remedy is to replace the system board, but I was hoping for something simpler and less expensive. Anyone?