"I did saw him" is a grammatically correct sentence, but it has nothing to do with "seeing." Are you familiar with the verb "saw"? It means to cut or divide with a saw. I doubt that is what you intended to express.
The auxiliary verb 'do' is used to emphasize the main verb 'see'. In this case it's emphasizing a past event, but this structure can be used in a present tense situation; e.g. I do want to go the party. / She does need some help.