In#2, you saw him while he was in the middle of the action.
As the taking of a bike is normally an action of short duration, there is not a big difference in meaning between the two.
However, there is a real difference between these two:
3. I saw him run a marathon.
4. I saw him running a marathon.
In #3, you saw the whole two or three hours of the running.
In #4, you saw only a part, possibly a small part.
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