On the other hand, Stiles is also known for his quick thinking. He might start to try and bargain with Peter. He could offer to help Peter in some way if Peter lets him go. For example, if there's a problem in the supernatural community that Peter is facing, Stiles could say he'll use his human ingenuity to solve it. Also, Stiles could be angry. He might start shouting at Peter, calling him out on his actions, and trying to make Peter see how wrong this is.
Stiles usually reacts with a mix of fear and defiance. He's afraid because he's in a dangerous situation, but his strong - willed nature makes him defiant. For example, in many fanfictions, he'll shout at Peter, trying to show that he won't be easily broken.
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.
Some of his friends might be really surprised. Scott, for example, might be a bit worried that Stiles' vampire obsession could lead him into dangerous situations. Lydia, on the other hand, could be more intrigued and might want to know more about the fanfiction. She might see it as an interesting intellectual pursuit.
Derek could be initially hesitant. He has his own past traumas and might worry about how he could be a part of the child's life. However, as he sees Stiles struggling to handle it all alone, his feelings for Stiles would push him to become more involved. He would start to see the son as a new beginning, a chance to build a family in a way he never thought possible before. He'd offer his werewolf skills to keep the child safe and teach him about the supernatural world if needed.
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.
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.
Scott would probably be very protective. He'd feel responsible for taking care of Stiles and making sure he doesn't get into too much trouble. Lydia might be initially shocked but then use her intelligence to try and figure out how to reverse the situation. And Derek, well, he'd probably be a bit annoyed at first but would also end up being one of Stiles' protectors.
His friends would be devastated. Scott, for example, might go into a state of extreme anger and blame himself for not being able to protect Stiles. Lydia could be inconsolable, using her banshee powers to try and find some connection to Stiles in the afterlife.
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.