I would put in a comma in the sentence to make it a tag question.
I couldn't not help him, right?
So by making this statement you are seeking confirmation from someone else that you were powerless to do anything for "him". It is not certain whether you actually tried to help the person or just gave up after thinking about his problem.
You could say:
"I was powerless to help him, right?"
"I tried to help him, right?"
"It was impossible to help him, right"
EDIT: As pointed out below I failed to read the double negative.
So I would offer simply
"I had to help him, right?"
I hate using double negatives... too easy to miss if you are not careful.