It's a tricky thing to do - use something as cliched as "boo hoo hoo", or try to write the sound, like "uh huuh huuh"? (My example could also be read as laughter).
Perhaps it's better to follow meow's suggestion and describe how the dialogue is presented:
"I'm sorry!" he said, crying.
"I'm sorry!" he cried tearfully.
"I'm sorry!" he sobbed.