His problems have nothing to do with the blue whale or any other fetishes.
You need to be concerned about the poor grades and the fights. Make an appointment with his school counselor and find out what is going on.
Are you kidding when you write "We don't want our son to be involved with things this heinous. This could be very hard on social relations in the future"? Look, kids at 14 are horny, and as my husband said about that time period, "I would get an erection if someone just looked at my d ong cross-eyed." Your son has found something that feels a little like a body, and he wants to use it to masturbate. I don't think it's a fetish, and I'm SURE that when he finally grows up and can have a regular sex life, he will not be dreaming of blue whales. You're the dad, buy him a package of white tube socks and tell him that he should use these rather than the sheets because they're easier to wash, and leave it at that.
Okay, first off I would recommend trying to worry about the grades and defiant behavior before worrying about him having any sort of "fetish". I remember when I was 13 and starting to go through this stage of puberty, and it really was a confusing ordeal to deal with.
You think that you have day to day life figured out, and suddenly new thoughts and feelings start affecting you out of the blue. Urges that drive you to do things that you have never done before. You start thinking about things you have never thought about before. And for some (like me) it can be difficult to go through these changes and keep your grades up in school.
Your son is becoming a man. The details in this crucial step in the process is so easily forgotten when you take into consideration that there are many steps after this leading to adulthood.
As far as your worry about such actions being a "fetish", I wouldn't worry about it too much. Those brand new feelings affecting him are going to make him act a little differently and do odd things, but they will pass. If anything can be done, "having the talk" can help some kids relax and not get really anxious about those feelings. Even if they look miserable while you are speaking about it, they are still taking in what you say.
I hope this helps you out, if only to give you some comfort in knowing that your son is simply trying to deal with his body changing, nothing more.
I agree with Rock Rose, Emily that was my first thought that this is a troll and as far as the regulars here are concerned we do know when folks are speaking in English and are from another country / culture, I have been aware many times as have others ..and of course I could also be wrong .....
His problems in school are separate from his liking of his inflatable whale. The grades and problems in school need to be addressed by talking to his teachers and seeing where he has room for improvement and possibly getting him some class tutoring.
His involvement with the plastic whale is completely normal for a 14 year old boy. He is at an age where he is figuring out his new hormones and feelings and his whale toy has given him an outlet to express them. In the large scheme of things, it's better that he is exploring and enjoying his inflatable whale rather than engaging in risky behavior with girls at his age. It is best to leave him alone and let him do his thing when he is with his inflatable whale and he will grow out of it eventually.