They beat him till he fell to his knees.
They beat him till he fell on his knees.
What's the difference between the two? Are both of them grammatically correct?
I would use:
They beat him till he fell to his knees.
since it is an ending state that is kneeling.
A better example of using on might be:
He lost his grip and fell on his knees.
where the meaning is actually falling and striking his knees.
If you are talking about the idiom, the FreeDictionaries says that both are correct.
fall on one's knees and fall to one's knees
means -
to kneel down, usually in respect.