I'm not defending any of the characters but in my opinion the way Aditya acts isn't way off mark. There are a lot of posts regarding why Aditya has a problem with Raman and Ishita and not Shagun and Ashok. Also someone questioned why Shagun has no problems being motherly towards Aditya versus being motherly towards Ruhi. There seems to be a disconnect there for most but on the whole the dynamics aren't completely ill-conceived.
Based on Shagun's personality she's materialistic. She knows she is beautiful and flaunts it. So earning love and adoration don't mean much to her, she wants the glamour and status to go along with it which is why she prefers Ashok. It doesn't mean she's not capable of loving or giving love, it's just that for her love equates to materials.
Based on the glimpses of what we've seen of their marriage, Raman and Shagun weren't completely unhappy in the beginning of their marriage,and Raman and Adi seemed to share a connection. Raman loved both Adi and Shagun with all of his heart (he still loves Adi). Based on that you can tell that Adi must have adored his father as well in the initial stages.
Most kids start out viewing their parents as people who can do no wrong till they grow up and realize that even parents can have faults and may not know the answer to everything. Adi's falling out with Raman begins with the constant fighting between Shagun and Raman in the later years. Shagun makes her displeasure known and based on what we've seen has taught her son that Raman is incompetent. He can't provide things for them the way Ashok can. Now Adi sees Raman as this man who can't provide for him or keep him and his mom happy. There's a slow resentment starting to build. Adi's being spoiled by his mother while his father is strict with him scolding him when he does things wrong. In a kids mind, fueled by Shagun's feelings Raman is the bad guy. Raman is keeping his mom and himself from being happy while Ashok does the opposite. So over time with that constantly repeated to him it's normal for him to hate Raman and Raman's family.
Aditya has become more a mirror of his mother who has spoiled him. He's more pleased with gifts and for him affection is won by who can give him the most expensive gift. Here Ashok. It's also why he doesn't seem to push Ashok away the way he seems to resent Ishita. Because Ashok has been there and providing for him while his own dad didn't.
This is also why Shagun can provide for Aditya easily than Ruhi. Ruhi requires love. Materials such as toys and fancy parties don't mean much to her as long as she has the care and attention that Ishita provides to her. Ishita listens to her. Shagun can leave Aditya with video games and do her own things, whereas Ruhi is lonely and requires company.
Aditya is lonely too and he wants affection but he doesn't understand the concept. He craves attention from Raman but doesn't know it and any ill-feelings Adi has he blames Raman for when in truth, it's his mother who's created the divide. Instead Aditya resents Raman for being the reason for all the unhappiness, so he wants to taunt and see Raman suffer. These are very basic feelings we see on the surface. The resentment and anger.
So seeing Raman happy being with Ishita is like a blow. Aditya might have all the possessions in the world but he doesn't know how to be happy (not in the way Ruhi can). Since those things can't really give you the kind of happiness a person needs. There was a time he did but he doesn't anymore, and for him Raman is the one to blame. Which is why he doesn't care for Ishita. He's at the age where everything is in relation to him not anyone else, and Ishita and Raman to him only build on the anger that his mother and Ashok have harvested over time.
He might be older to Ruhi but his view on relationships and happiness, he understands less than Ruhi. She's in a way much wiser than he is, even unintentionally because it's just the way she's been brought up.
Raman is reaching out to Adi to convince he's not the bad guy and it's not that he loves Adi more than Ruhi, it's just that because Ruhi loves him, he can easily take it for granted. Raman still very much things and acts with his heart despite he telling himself otherwise. He's not very good with thinking things and seeing the bigger picture and hence makes really bad choices. That's why even when he takes a step forward, it's easy for Shagun and Ashok to bring him Two steps back.
One can't really ask him to stop fighting for Adi and focus on Ruhi because both are his kids, but he will have to eventually realize that instead of trying to placate Adi it's best to be brutally honest and let the kid come on his own.
He also has to learn that people around him are not punching bags of his emotion, especially if he only shares his anger. If Raman had shared with Ishita why he couldn't have her there then she would have complied but as always Raman seems to be one-sided with his emotions. If he wants to keep his feelings bottled then he needs to keep his anger there as well.
Sorry for the long post just wanted to share what I see in dynamics of the relationship.