No contract or title transfer is valid if signed under duress. The only thing the robbers did was establish a paperwork trail that will probably get them caught. So, don't worry about the title. If the police didn't do this already, and they probably did, simply go to the DMV with the police report and have them flag the registration.
I'd love to know the full story. Did they hold him at gunpoint and make him sign the title? Did they just wait around while he went and found the title? Were they actually trying to buy the bike and then not pay for it? Is it someone he knows?
Definitely go to the police. The VIN will come up stolen when they try to transfer the title.
"We didn't trade shoves; I did all the shoving." -Bobby Knight, 1981
they came to the house because my dad was trying to sell his bike, he looked at the bike for a while then he asked my dad can he hear it then he hit the throttle a couple times the pulled out a gun and said "lets go inside and get the titles and sign them" so he followed him inside to get the titles my dad signed them them he told my dad to get in the car because he is about to take a ride with them so he can go drop him off on very long and dark street to give them enough time to go hide the bike