He will start to see something. Reactions to epiphany vary. But "fully gaining sight" will be difficult.
There is one thought experiment called the Molyneux Problem. The bottom line: a person, blind from birth, learned to clearly define with his fingers what kind of figure in front of him is a ball or a cube. If he was miraculously given sight, could he, with the help of his eyes alone, again determine the shape of the figures in front of him?
Real life experiences so far indicate that it will be difficult for a person to see the light to solve the Molyneux Problem.
Brains must be able to see with their eyes so that this can be called "fully acquired vision." Full vision cannot be simply turned on at any time in life. For most sighted people, this takes years of training as a child. In addition to the eyes themselves, other senses help to learn to see. The child runs and falls / crashes into an obstacle - this is how he learns, including vision. A blow to the knees / forehead is important information that will link with information received from the ears and eyes, and certain conclusions about the world will be drawn - useful connections in neurons will appear / strengthen.
The earlier a person " turn on vision ", the more effectively he learns to see, the more necessary and convenient visual information about the world will be for him.
Depends on its adaptation. If everything is in order with his hearing, he learned to move, write, read special fonts such as barail, social services, VOS help him, then gaining sight will make him very happy for a long time, he will, as if on wings, conquer new and new peaks.
If a person is deaf and blind from birth, then unfortunately such people do not develop. They even lose their orientation reflex / instinct. Their brains don't develop. They have almost no chance of adapting. If then he regains his vision, then with such a lazy brain, it will be more difficult than with a newborn.
With an average adaptation, there will be something in between. The less adaptability, the more stress there will be.
The state of modern science and technology is still far from perfect. There are bionic implants, for people who have almost lost their sight, Argus II, but the picture they give out is not much better than Minecraft, and it is adapted for people with certain pathologies. And it costs - quite a lot, the operations are piece have not been observed before, because the effect is not so significant.
If not ruling, then we can assume that it will look something like this: https: //www.youtube.com/watch? v = 5uPQxAZSzvk
In the case of the blind from birth, there will probably be a lot of confusion at first. These people do not know how and what they look like, are not able to separate visual objects and their boundaries in their consciousness. Perhaps it will be great, beautiful, new sensations, although it is not clear what corresponds to what in this new world. We'll have to learn to live in a new way. Something similar can be observed in people who first tried on VR glasses, who tried to play a video game. The discrepancy between visual data and all others - receptors that determine the position in space, the state of the muscles, and the environment - all this is new, unusual, and at first it can even shock. Or, there are still such experiences when a person in VR glasses is broadcasting a video from a video camera fixed on a tripod, for example, behind his back, as if from a third person. Or, another tricky option - a person controls the robot remotely (the same VR glasses), and the camera is on the robot's head. And the person comes up to the robot, and shakes hands with himself, seeing it from an unusual angle. These are approximately the same unusual sensations that blind people from birth may experience when they gain sight.