Someone asked me whether there was a difference between 'I saw him walk along the road' and 'I saw him walking along the road'. Both are grammatically correct, but there is a slight difference in meaning. When saw (or another verb of perception eg hear, notice) is followed by the base form of the verb, or the infinitive without 'to' (as 'walk' in the sentence above) the emphasis is on the result of the action. When the -ing form is used the emphasis is on the process, or the continuation of the action.
The difference is perhaps clearer in the sentences 'I saw him do it' and 'I saw him doing it', and it is clearer still in 'I saw him drowning but he was rescued just in time'. You definitely need the -ing form here; 'I saw him drown but he was rescued just in time' is not a possible sentence, as the form 'drown' shows he couldn't possibly have been rescued.