Hiccup’s wingsuit is tailless. It is technically just a pair of wings, nothing else.
So don’t be surprised if your flights end up in huge failures, Hiccup.
The thing about tailless aircraft is that they are prone to instability in pitch. With conventional airfoils at their usual placements, their center of pressure (where you apply the net lift) would be usually closer to the nose of the plane than the center of mass/gravity of the plane would be. What this means is that even if the pilot manages to get it to cruise horizontally, any torque that causes the nose of the plane to point up will cause the wings to generate the same kind of torque as well. In other words, the plane would turn increasingly nose-up with little opportunity for the pilot to intervene.
What the tailplane (like Toothless’s fins) provides is the similar torque at the opposite end of the plane. As the plane goes nose-up, the upward torque generated by the main wings increases, but so does the upward torque generated by the tailplane, applied at the opposite end. Thanks to the tailplane, there is significantly less net torque to deal with, making the plane much more stable.
Well, let’s look at Hiccup’s wingsuit. There’s no sort of a tailplane, unlike the wingsuits that are commonly used today. With tailless aircraft, engineers usually compensate for the lack of a horizontal stabilizer (tailplane) by using swept wings or delta wings, which would move the center of lift pretty much to the center of gravity, reducing net torque (Torque = distance from center of rotation x force applied). However, Hiccup is using wings that would not accomplish this, and still his feet are getting little lift.
With all that, my hypothesis on how a typical flight might end up: Hiccup’s legs drop when he can’t keep them up with sheer force anymore. The wings follow. Hiccup turns head-up, he stalls, he falls (was there a rhyme somewhere in that sentence?). TOOTHLESS!
This is not a pitch-perfect, accurate analysis. Let’s leave that up to the people who actually can do multivariable calculus, and own a wind tunnel.