1. If I were to see him, I'll tell you.
Almost every kind of "mixed" conditional can be heard in spoken English, and found in informal written English (e.g. in emails, or on websites). So although it sounds incongruous to me, it wouldn't surprise me to encounter it. I would interpret it as a (probably unconscious) change of attitude on the speaker's part, between clauses.
In edited English (newspapers, books, etc.), on the other hand, it would probably be amended by an editor; moreover, the originator would probably accept the amendment quite happily, if you suggested it to him.
In an exam, a student would probably be wise to change it to the blameless "If I see him".
Student or Learner