This situation seems too absurd to me. Before I say anything else I have to point out how problematic this sentence is: "I'm not a strong person so I can't beat him up."
Don't get me wrong, I'm not a person that thinks violence is never a solution. Sometimes it is, but it is never a solution if you are not in danger of physical harm. Let's say that you are strong and you know how to fight. He comes and bumps on your chair, you get up, you start punching him, you drop him on the floor and you punch him some more. How do you justify yourself? How can you claim this was a reasonable reaction?
This is a common misconception with martial arts. People think that if they train in some martial art, they will stop being pushovers. But it doesn't work like that. Martial art will train skills that have no place in such situations.
So, he is bullying you because you make yourself an easy target. He comes and splashes you with water and you laugh. This shows him that there will be no retaliation. This needs to change.
Think why is he doing that? Is it because he feels insecure? Being the boss's son is not necessarily easy. Especially if your father is tough with you. Does he want to show that he is tough? You know the situation better, you should be able to guess. Once you know what his need is, you can use it against him.
Think of offensive psychology. He splashes you with water, you look at him with contempt and you start asking him how old is he, what is his mental age, whether he thinks that will make others respect him, whether he thinks people respect spoilt brats, etc. Then you go around telling people how he splashes water on you and that you are shocked that someone can be so immature. You didn't do anything wrong, you have nothing to feel embarrassed about.
I know these things do not come up easily. You probably grew up trying to be a good kid and being offensive to others is a taboo. Unfortunately, no one will give you a cookie for that. You need to give this cookie to yourself.
There are a couple of caveats you should be careful with:
Never lose your cool. Always talk calmly, but with contempt.
Do not be the bully. Only criticise his behaviour and nothing else. You should criticise only what he chooses to do.
Stop as soon as he stops bothering you. You want to go for the "do not kick a sleeping dog" effect.