His sweat is acidic to his physiology so if he exerts himself too much then his skin will start to burn, and there's nothing he can do to stop the sensation and pain so he'll have a heart rate watch that uses a percentage (0 being he's good to go, 100 being he's up shit creek) that i would incorporate into the story. For example, for every minute of using super strength it would go up a percentage, but it would also apply to mental powers as well as it forces him to concentrate and use metabolism, too much power usage is not good for him so he has to pick and choose his battles. Stopping a normal guy robbing a bank, no biggie, stopping a supervillain of some kind from city wide destruction, he has to be smart about it. If there's an alien invasion and he has to strain himself going every which way, he wouldnt be any help to people if he was burning alive