Are you two on a domain by chance? like it was mentioned above, if you both are on a domain and have domain admin levels of access he can go into computer managment and make changes like re enabling remote registry and good stuff like that.

Although, it seems kind of lame to me that you are here asking for help in the war hehe. Just please be careful, I have participated in a more than a few of these (both in school and at work) and people tend to take things too seriously sometimes, and real damage can happen both on purpose and by mistake. well have fun, and start getting more creative with your attacks, if you get him good enough he will be too busy defending himself to attack back.