In the bit where Lieutenant Dan's wheelchair takes him down the ramp backwards super-fast just after talking to Forrest, you can clearly see that as he zooms past the camera, he has legs.
I did some research and found out that Gary Sinise had strongly opposed to having his legs actually amputated for the sake of staying with the realistic theme of the movie.
What's more is that a blind man could see that it's no leg at all. His pants are cut and tied off. What looks like a foot is the flaring of the part that is tied off. The reason the "amputated" leg is extended is to balance his fall in the wheel chair. Watch it closely. They spent millions on the special effects for his legs, I doubt they'd miss even one frame. Correct me if you think I'm wrong, but I submit this in all factuality.