Avant que always needs the subjunctive.

The subjunctive has nothing to do with formality. The best example of this is hearing a teenager announce to his friends il faut que je m’en aille.

Although the ne explétif isn’t obligatory according to some, I would use it in your sentence.

It’s syntactic role is to emphasize semi-negation. Action not being held/never going to be held.

Full sentence would be :
[…] avant que je ne le voie.