How can you keep them from being overpowered, hmm - how about letting them control only one element at a time. Still pretty dangerous, especially if they've time to plan - but if they have to let go of whatever they're doing, to counter (or even sense) what's coming at them, they are less dangerous. If they're matched by more than one person, they can have their attention divided even if the sum of their talents matches the sum of their opponents' talents, because they can only do one thing at a time (even if it is a great and planned thing). One set of eyes, one mind, one thing at a time.
What else... You can go the standard route of having the powers (physically) expensive, have them run out of energy after a while, and need food and water and rest. They will need to concentrate in order to do things - and having more power to keep track of, they might find it harder to concentrate than a single-element. Possibly, they might have more sensory input depending on how "connected" they are to each element - if they can sense the element, and/or tell if someone nearby is manipulating or attacking with it - they can sense ALL OF EVERYTHING and thus are easily distractable, even though that was originally a good skill.
Or, if each elemental power has its upsides and downsides, strengths and weaknesses (as each power should, to be balanced) have him hit by all the downsides as well as the upsides. Even if only when they're using the element in question, it might help bring them down in power by enough. Or make using more than one element at a time make it possible for the elements and/or their limitations, or even their boosts, to clash with each other or possibly cancel each other out. If the power affects the person as much as the person wields the power (as some stories have it) then having different energies flow through at different times, or having them competing, might be very troublesome indeed.
I can't tell you how they got the power, it is your world and story. But, it was probably really dangerous, if no one else has succeeded or even tried. Maybe he bargained with other powers or spirits, or risked his life, went on spiritual quests, or made sacrifices. He may have extra downsides attached to the method that let him get control, beyond the power limitations themselves - if he has bargained, he is limited by the agreement he made with whatever bargained with him (including limitations in power or behavior). If he risked his life doing something risky, (again, risky enough no one ever has done so) he may have physical damage, old wounds, psychological damage or power limitations from whatever it cost him to survive.
Can the powers be taken back... yes. If he bargained with something, they can take it back (if properly appealed). Not the same thing as saying they must, though, it will take cause and depend on the bargain. If it was a spell, there's a counter-spell. If all else fails, try brain damage - that will stop just about anything. It might leave him unable to draw the power, or make him forget he has it, or not let him concentrate enough to use it. It can be targeted (spell of forgetting), it can be massive (hit him over the head till he stops moving, which can leave any damage), it can be clinical and targeted (brain surgery), it can be violent (spike through his skull, if he survives - not gonna be controlling anything).