He can't control his heart either. It thumped wildly in his chest. If his heart were not beating so fast, then he won't have this problem of having too much energy and being unable to control it. Unfortunately, his heart is beating so fast because of all the adrenaline flowing through his body. He can't control that too. Adrenaline is flowing so much because he is afraid.
He is in fight or flight mode. It culminates in having too much energy in his body. The energy within his body threatened to go out of control. No one should wield energy like this. It can damage his body. It is not safe, and yet he is pumping even more Mana through his body.
He held on to his energy by sheer force of will. There's a lot he can't control already. He can't control his heart, and he can't control his fear, but he will be damned if he doesn't control the energy within his body.
