Hi C.,
This is SO typical of the oldest child, and most likely has nothing to do with your husband being gone. Partially, it's just because they recognize their "status" as being older. "Mom is older than me and tells ME what to do, I'm older than my little brother, so I tell HIM what to do." My son did the same thing, and I did it when I was a kid!
But, here are two things that might really help.
First: A good discussion with your son is probably all that's really needed. Nothing nasty or critical of course, but just an explanation. Explain to him that only mom and dad discipline. And also, that only mom and dad give orders, assign chores, etc. You'll need to remind him each time he does it so that he becomes aware-- point out "sweety, you're bossing again. Mommy's right here and I'll handle it."
Second: Be careful that you don't confuse him. Oldest children get confused sometimes, because often, without realizing it, we parents give conflicting directions! (Or at least directions that SEEM conflicting to a young child.)
.
"Watch your sister for a minute while I go in the basement to do the laundry",
"You're the big brother (or big sister)you need to set a good example."
"Help your little sister do this or that"
"Go check on your brother"
"Tell your brother to stop banging, I'm on the phone"
etc, etc, etc
There are lots of times when we make them feel like they ARE responsible or DO have some kind of charge over they're younger siblings. Then we get upset or frustrated when they take the initiative to intervene on their own. They don't know or understand where the boundaries are -- where their responsibility starts and where it ends.
I don't think it means that we can't ever ask an older child to help with a younger one of course, but we need to be cautious that we aren't sending mixed messages.
So making it clear that Mommy and Daddy are "in charge", but that SOMETIMES, mom and dad might give you an 'assignment'to check on a younger brother or sister, to keep an eye on them and make sure they're safe while we use the restroom, run downstairs to throw a load in the dryer, etc -- but that's all it is- an assignment. They should ALWAYS look out for them safety wise, (like if mommy's in the bathroom and baby sister found a pair of scissors) but bossing them around, trying to punish, or discipline is Mom and Dad's job.
Blessings!
T.