Look around for a box with a glass window, there should be a rocket launcher inside. When you have it, look around for switches that turn off the lights, and while you and Wesker are in the dark, fire the rocket at him, and he should catch it. While he is holding it, shoot it, which should stun him, then approach him and the command "Restrain" should appear on the screen. If this works, Sheva should inject him with the syringe acquired from Excella earlier.
If you need a better explanation than this, and you have an xbox 360, my gamertag is Lolbot 3597. Add me, and I'll be able to help you directly. If not, you can talk to me using hotmail, my address is firstname.lastname@example.org.