I almost want to say this is one giant troll. Almost.
But I have a little more faith in humanity than that, so I am going with my alternate theory. What if the OP does not actually have this issue, but instead came up with this hypothetical scenario to read people's reactions? OR... What if the OP is actually the grandfather and is trying to see if it would be okay to go rape his grandson's hair with shampoo?
Now for a more on-topic response...
Personal hygiene is just that. It is PERSONAL. Ultimately, you choose how clean you wish to be, and no one else can choose that for you. Your grandfather is not in the right, here. He can, by no ethical means, determine your level of hygiene for you.
Now, assuming you are under 18 and he has legal guardianship of you, then he can hand out punishment after punishment until you bend to his will and clean yourself (he could even take away your internets!). If you ARE 18 (or are legally emancipated), then he can simply throw you out because it IS his house, not yours.
Take, for example, this scenario: I have a co-worker who never wears deodorant and always smells terrible. My boss told him that he needs to start wearing deodorant as his smell is offensive. In this situation, my boss cannot put deodorant in my co-worker's armpits. However, if my co-worker refuses to comply, it constitutes a violation of company policy and he can be fired. Simple as that.
So I hold no sympathy for you. If it as simple as using a better shampoo and showering every day, then I suggest you grow up. Welcome to the real world.
Of course, if it is worse than that, then seek professional advice. I absolutely understand that Head and Shoulders might not do the job for you, but... You should at least be making an effort to be clean (if not for your sake, then for the sanity of those around you!)