I feel like it would have started when he was ill one time and realized that a certain tone of voice would get him just anything from Harry and even a little extra pampering.
It was good enough to make him pretend like he was still sick when in reality he felt pretty good already. He just never wanted it to stop. He always wanted to be able to roll on his side and whimper until Harry came over and started rubbing his belly; or make silent grabby hands at him and not be chastised for being too childish.
Eventually he did need to stop being sick, though, and the following two weeks were some of the worst in his life because he had just such a sudden need for being pampered and someone tending to his every need. It took him a while to realize that the special treatment wasn’t a special thing just because he was sick, but that he could have it whenever he wanted.
That he just needed to pitch his voice just so and Harry’s face would fall – well, as much as that was possible with Harry – and he would get a little special attention.
He’d get his belly rubbed (and, subsequently, if he started rocking his hips into it, his cock), he’d get hand fed all those unhealthy treats he loved, he’d be able to stick his head beneath Harry’s sweater and rub his face against him, just as much as he liked (because Harry always smelled so fucking good)
And on one memorable, magical occasion, he even got carried upstairs; with Harry’s big, long fingered hands under his bottom, holding him – and rocking him just a little