He's a half saiyan who doesn't have the drive or determination that others possess due to the fact that he's a hybrid, or he was just not that into fighting/world saving. His "training" with Piccolo was basically just a easy day for kid Goku, camping in the wilderness. When they did start actual fight work sure he showed some skill, he's Goku's son. Doesn't change the fact that when it comes down to it, in every situation when he could have done something he either ran or was frozen with fear. For all the mystic power up did to he power level all it took to beat him was to get in a few blows then call him out for being pathetic.
Don't forget... Gohan was the first kid to stand up to Freeza... And live.