I think it would be too much of a penalty to prevent him from re-loading in melee. Is there anything else that does this? Of course, it would make him pull out other weapons and do different things in combat if I did. I'm also thinking of the game balance regarding casters. It would make casters that much more powerful if I didn't let people re-load in combat.
Well...yes. It's a fact of the setting that it's
modern weaponry that evens the odds between humans and the supernatural world. I'm all for game balance, but to me, letting someone reload a musket in melee just plain breaks suspension of disbelief, even with the speed power.
Quite frankly, a musket isn't the best choice for a weapon in close quarters, at least as far as firing it goes. That's a fact of the weapon, same as saying that a sword isn't a long-range weapon. If he wants to use a musket, this is one of those truths of the universe he's going to have to contend with--it is, quite simply, not something you can fire with any sort of rapidity. The nature of the weapon was something that every warfighting civilization had to adapt to, so he should too.
Look at every Civil War or Revolutionary War movie--at
most there are two, three volleys of musket fire, and you only have time for that because the other side is literally just standing there. Once the melee starts you just plain don't see musket fire because it is more or less impossible to reload a six-foot-long weapon while someone is swinging a bayonet at your face.
If you really have to "balance" it, balance it by compelling him repeatedly to not be able to fire if he's in the middle of dodging.