It doesn't sound like the boat tipped over if his belongings were found inside the boat. I wonder if he was reaching for something and flipped out of the boat? If there is junk in the water he may have gotten trapped under something and because of the cold was unable to get un-tangled. That is all I can come up with.
Doesn't sound like there was any reason that he would want to just disappear. Hopfully he will be found and is still alive. His poor parents and girlfriend.