Walk up and pick the option that beats his. Punish advances forward, limit his options.
Aerials he does before Fox reaches the Apex of his shorthop can be beaten OoS, pretty much guaranteed if you shield DI to follow where they're projected to land, and you can WD OoS when you don't know if you can punish in whatever direction he's least able to punish or gets you the best position.
If you remain close enough, he can't laser. If he's doing retreating aerials, he's cornering himself.
You can DD to try to whiff punish just about any actual attack.
Crouch cancel destroys jab, jab is the lowest commitment option to interrupt movement (i.e. dashdancing or anything relating to jump from too close). Otherwise, if you're more confident in hitting his position, grab or dsmash work vs the ground and shorthop.
If you corner him, it's easy to cover any attempt to go towards center stage and hard for him to punish use of moves that could knock him off the ledge.
Be patient yourself. Shield or dodge attacks, try to hit at least a few dashes. Expect him to try to fullhop or use platforms at some point if you can consistently stuff his ground game or make him question his positioning (or either double jump or waveland if he's shorthopping to a spot you're not at). Watch whatever he does in the air, you can usually punish on reaction.
Oh yeah, charge needles when he can't threaten you reasonably quickly and throw them if he lasers from mid distances, or use them as traps to deny space for him to use certain moves.
Work on quickly using ftilt and utilt, and exercise restraint in their use vs shields.
Grab is like, every character's best move. Sheik and Fox have good grabs and good grab games. Try to land yours. You can stuff his grab with moves that poke, like jab, or avoid them. AC fair can help vs a grab happy Fox, and vs dash attacks or aerials from slightly farther. Rising nair could also work, but beware of earlier percents.
Try to avoid wavedashing towards Fox if you don't have a near guaranteed punish. Expect him to be able to react to spotdodges, but if one isn't punished, try to either get out of that position as safely as possible, or attack. Also, avoid spotdodges if you don't expect to get a punish out of doing one, roll or WD would be preferable for defensive purposes unless you actually need invincibility on the second frame (if you're not in a pressure situation, this isn't likely).
