Though it may be hard to tell, Piccolo began growing attached to the kid from the start, watching over him and night and giving him apples, new clothes, and a sword. He limited his training time so he could go check on the kid.
The namekian trained Gohan brutally and mercilessly, knocking him off cliffs, into rivers, and leaving him with a face-full of bruises and cuts. But all the same, he developed a sense of pride in his pupil, telling himself the kid would be stronger than all the rest of the fighters.
When the battle came around, Piccolo was quickly disappointed by Gohan being a wimp against the saibamen and Nappa. Though he yelled at the child at first and turned his back on him, the namekian still cared about his safety. Enough to give his life for the child.
During the rest of the series, particuarly in the Freeza and Garlic Jr. Sagas, Piccolo is always the one to deflect blasts from the boy and kick away big bad guys. I honestly think he'd make a better father to Gohan than Goku. After all, who was the one that took MetaMacha's blast in movie 4, and later tried to keep Gohan from freezing to death. Not Goku, you fool! Piccolo!