How do you add a new player halfway through the season without changing the whole group?

I would make belonging the first target, not catching him up. If he walks into every session feeling like “the beginner”, he is going to compare himself with children who may have two or three years more experience. That is a difficult place to learn from. Keep him in the same games and group, but adjust the challenge quietly. Give him a bit more time or space when you can, pair him with patient teammates, and notice small successes that are genuinely his. I would not stop the whole group to reteach everything from the beginning for him. The stronger players can also learn from this. Playing with different teammates, helping without taking over and adapting their decisions are useful skills. Most importantly, avoid labels. He is a new teammate, not the weak player. If the environment feels safe and he gets lots of actual football, the technical gap often closes faster than adults expect.