There is not much you can do with how traffic is routed to dest from his connection. There is no way to change that.. It would come down to who the isp peers with, etc..
Going all the way to the US from AU to get to singapore sure seems like a the long way around
The only way to accomplish what your after is either a vpn to your box, or a proxy you run on your box. Depending what this game server is proxy might work, if you go with vpn. All traffic could be routed through the tunnel.
Yes no matter what vpn you use its going to add a bit of latency.. Should be all that much if your talking 400 from him, and your seeing 50 from him to you, and then 175 from you to the game servers. 175 still seems like a bit much from AU to Singapore - how are you running.. Can you post up your hops? Curious to see his hops as well.
I would be curious to see the RTT I get to this game server, I have a connection in Mainland china I could test with.
As stated your best option is vpn tunnel -- as to ipsec or openvpn or something else. Pick one that your more comfortable with or more familiar with or one you would like to learn and go with it. Should really be too much difference in latency between them not when your talking like 225 anyway
And looking to improve upon 400.