I screwed up my high school Freshman and Sophmore year, due to over-use of Counter-Strike.
Do not give it back to him--if you do, limit it to one hour of cs after he is done his homework.
Watch him do his work. Also beware he may say he doesnt have HW just so he can play CS, or do trickery on you. I suggest you help him with his work, aid him, and give him help when he needs it.
I wish I had a brother like you, I do have a brother--but he was in college and couldnt help me. I used to be just like your brother. Do not let him get away with it--do not give him his computer back.
If you do give him his box back, take his VID CARD OUT, and replace it with one that cannot handle HL2/CS:Source.
Make sure he is doing work only on it.
If he ends up like I did, it will be a sad day. Computers, and Gaming has faaked my life up, royally. Im doing Engineering now, trying to rectify my mistake, I hope it not too late.
If you do give it back--watch him do it, check over his HW, or even HELP him write papers and such; Not by doing it for him, but by reading it over, giving him pointers, tips, and even give him one liners to start off one. Get his mind flowing, help him out as much as possible, without it being non-legit.
Good luck, I really hope you will be able to straighten out your brother. Counter-Strike is overly addictive, and can ruin his life, among other things.
P.S--let him know, if he gets straight B's AND KEEPS IT UP, by balancing work and play--he can have unrestricted use of the PC. BUT THE SECOUND grades start slipping, remove the privledge.
P.P.S--Your bro may not like it, but you should keep in contact with his teachers, go in for meetings, and see what you can do to help him.
-Monzi