I read this novel in the early 1970s, but it may have been written as early as the 50s.
That's The Secret of the Ninth Planet by Donald A. Wollheim, first published in 1959. There is a Project Gutenberg etext. Here is the back cover blurb from the 1965 Paperback Library edition:
SOMEONE—OR SOMETHING—WAS STEALING THE SUN!
At first the theft was imperceptible—soon daylight seemed strangely dim. It was unseasonably cold Then the growing darkness and increasing frost posed a real threat to the survival of Earth! The world was alarmed and the hunt was on.
The solar system had to be searched. And because Burl Denning was close to the scene of the first disturbance, he was picked to go on the Magellan, the most incredible spaceship in the world. On planet after planet, Burl discovered Sun-Tap stations, strange machines that were draining the sun's energies away from Earth. The monstrous mechanisms had to be stopped before the world froze to death!
It was probably a hard-science,
For some value of "hard science" that includes anti-gravity drive spaceships.
Yep, originally published in the Winston Science Fiction series.
the protagonist was a boy,
Burl Denning, a senior in high school.
and it showcased what we knew of the solar system at the time.
The tour of the solar system included stops at tidelocked Mercury, watery Venus, Mars with canals and cities maintained by an insect civilization, Callisto, Iapetus, Oberon, Pluto, and Triton.
As I recall the plot, it began with the boy (and maybe his father) finding some abandoned alien technology either on earth or the moon.
Burl and his father, on vacation in Peru, go to the site of the alien machine at the request of the USAF. after it had been spotted from space. The aliens have not exactly abandoned it, though; it is unattended but in nefarious operation until Burl turns it off.
Nothing works until the boy touches one device, which gives him a shock.
As his hands touched the metallic ball, there was a sudden terrible flash of power. He felt himself grasped by forces beyond his control, paralyzed momentarily like one who has laid hold of an electrically charged wire. He opened his mouth to scream in agony, but he could say nothing. A great force surged through his body, radiating, charging every cell and atom of his being. He felt as if he were being lifted from the floor. Then the globe seemed to dissolve in his hands. It became a glare of light, grew misty, and then vanished.
After that, the boy finds that he can operate everything.
After resting a moment, he decided to try the useless controls again. Going over to one small board, he idly shoved a lever. This time he felt resistance. The lever was activated. There was a slight change in the radiance of one globe.
"Dad!" Burl shouted. "It works! It works for me now!"
Mark Denning watched as Burl turned dials and levers and got responses. "You must have been charged in a special way," he said excitedly. "That's how they lock their devices. They will only respond to a person carrying that special energy charge, whatever it was. Come on, let's get to the main control, before the effect goes away—if it does."
There is an encounter with some bad guys who I think are evil aliens, who are fought off.
No, the first direct encounter with bad guys is in the outer solar system.
The authorities are brought in—the boy's father is some kind of scientist or government official—and they determine that the bad guys are trying to make the sun darker, or blow up the world, or something equally bad.
"It is apparent that some race of beings, some species from outer space, unknown to us, has begun a process of tapping the power and light of the Sun for transmission elsewhere. The station on Earth, which you shut down, was an important one. But . . . it was not the only one. There are others, operating in this solar system." He nodded to Merckmann.
[. . .]
"Measurements of the amount of Sun power being piped away, and of the effect of the magnetic disturbances used to create and maintain these stations, have shown that they will have a definite effect on the structure of the Sun itself. We have not yet completed all our calculations, but preliminary studies indicate that if this type of solar interference is not stopped, it may cause our Sun to nova in somewhere between two and three years time."
The good guys take off in a spaceship with the boy and his father.
No, the boy goes on the mission, but the father stays behind.
The big finale takes place on Pluto, the ninth planet. The bad guys put up a last-ditch fight to stop the good guys. The boy's energy field has faded, and he has to take off his spacesuit gloves to operate the machine.
Actually, that scene takes place on Triton:
In a covered panel right next to the door, Burl found the typical Sun-tap controls. He tried to work them, but they would not function through his gloves.
He hesitated, knowing that removing his glove this time might prove very risky. Then he hastily drew off his left gauntlet and the thin nylon glove that was the inner protection of his suit. He placed his hand on the control. The icy cold bit into it. He twisted, the control worked, and he tore his hand away, replacing the gloves.