Mod it.
Ok, look, understand the complexity you're interacting with here. If you give him access to the same promotion, you'll only make it so that the units he commands gets the movement and combat modifier.
You COULD make it so that movement modifiers don't affect the units and instead affect the commander. But if you then open up the Quick March status to the commander, all his commanded units would suffer the combat modifier penalty even if they aren't using the status themselves and if they ARE using it AND the commander is, that would total to -100% combat modifier right? Wow.
If you then take combat modifiers given to the commander away from the units he commands then you've taken away most of the whole point of having a commander.
You would have to build entirely new tags to let the commander have these affects work for him alone and it's simply too much data to spend on the issue. However, if we make it so that some units can carry the commander with them then that solves the whole thing with a little extra AI work.
The situation is not as simple as it may seem because the way commanders work is to give their bonuses to the units they command through the same modifier tags as promotions normally give the unit itself the same ability. Aka the tag iCombatPercentage (or whatever it actually is) applied to a promotion given to a normal unit would add to the combat modifier of the unit, whereas the SAME tag given to a commander doesn't actually affect the commander at all but rather the bonus passes through to the troops he commands. If you wanted to do this for the commander himself you'd need an entirely new tag that doesn't pass its values to units under command. This then makes the whole mechanism's efforts to be data efficient completely moot.
There's too many other things to spend our data on and I'll be needing to make room at some point soon for more things and hopefully to shave enough data on units to cut it down overall by the time we're said and done with units in C2C. Adding things like this would be frivolous if we can solve it another way that doesn't cost the data.