Stiles might try to act like it doesn't matter at first. He'd put on a brave face and say something like 'It's no big deal' but then as time goes on, little things would start to show his disappointment. For instance, he might be a bit more sarcastic than usual or seem less enthusiastic about helping the pack with their problems.
He could also be angry. After all, he's always there for the pack. He'd feel like they don't value him as much as he values them. So he might confront them about it, yelling something like 'You guys are so self - centered! How could you forget my birthday?' And this could lead to a big argument within the pack that would force them to reevaluate their relationships.
He might be really hurt. Stiles is a sensitive guy deep down, and having his friends, who are like family to him, forget his birthday would sting. He could go into a bit of a sulk and isolate himself for a while.
It could also be that they are used to Stiles being the one who takes care of them and remembers things for them. He's always been the planner and the one to keep things in order. So they unconsciously assume that he doesn't really care about his own birthday as much as he does about helping them, and they forget all about it.
Some members of the pack might be angry at first. They could see his leaving as a betrayal, especially if they are in the middle of a difficult situation and rely on him in some way.
Stiles might initially be shocked. He has been a part of the pack for a long time, and Scott kicking him out would be completely unexpected. He could stand there in disbelief, unable to process what has just happened.
Stiles might be initially shocked. He would probably feel betrayed since he thought of the pack as his family. He could go into a state of denial at first, not believing that they would actually kick him out.
His father could be one of the supporters. His father has always been there for Stiles, and he would understand if Stiles was standing up for what's right within the pack. Another possibility is Lydia. She's a smart and powerful character herself, and she might see the value in Stiles' actions and support him.
Scott would be extremely worried and immediately start planning a rescue mission. He'd be angry at Theo for targeting Stiles and would use his alpha powers to try and track him down. He might also call on the rest of the pack for help, like Lydia, who could use her banshee powers to sense if Stiles was in danger or where he might be.
One way is for the pack members to stand up for Stiles more often. If someone outside the pack is giving him a hard time, they should be there to protect him. Also, they could share their secrets with him, which would show that they trust him. This trust is an important part of any relationship and would help Stiles realize that they don't actually hate him.
There could have been some misunderstandings. Perhaps Stiles overheard a conversation that he misinterpreted. Or some actions of the pack members were misconstrued by Stiles. For instance, if they were joking around but Stiles took it the wrong way and thought they were being mean on purpose, which made him think they hated him.
Well, Naruto could initially be shocked that no one remembers. But then he might try to play it cool, because that's his nature. He could also think that they might be planning a surprise later. However, as time passes and still no one remembers, he would start to feel really down. He might even go off on his own to brood about it, wondering if he's not as important to his friends as he thought.
The other characters might initially be confused. They could wonder why Stiles is suddenly absent and start to question if they did something wrong. Some characters may feel a sense of loss, especially those who were close to Stiles in the pack. They might miss his input during the meetings as he often had unique perspectives.