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.
There could be a mix of reactions. Some in the pack might worry that Ayame's pregnancy could make her a target. So they increase security around her. Others might be overjoyed and start planning big celebrations in advance. The pack's reaction would add an interesting layer to the fanfiction as it shows different personalities and their relationships within the group.
On the other hand, she could see it as an opportunity for a new start. She might gather her strength and decide to prove herself outside of the pack, or she could try to find a new group or community where she would be more accepted.
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.
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.
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.