Opening Ports on one computer linked 2 others

Hello,
I'm trying to figure out how to isolate my computer from my local network. We have 3 computers in the house and my little brother is trying to play a game that utilizes the internet. We have firewalls and router blocks that slow down his gameplay and connection. How can i set his computer outside the network to access the ports he needs and still use the same IP address? Also, i need to keep the other computers locked down; leaving the ports locked for their internet use.