Okay, I know I'm only 12 weeks pregnant but it's fun to think of baby names so give me a little slack :)
Our first child's name is Oliver James
If we have a girl, we are pretty settled on Elizabeth Jane
So now... What if we have another boy?
As you can see we like the older names. I THINK we are going to go with Thomas for a first name (My husband's thoughts on this change every time I ask him, but for right now it's what I'm going to work with).
What middle names go with Thomas?
My nephew is Thomas Christopher and my sister’s nephew-in-law is Thomas Joseph.
I like Thomas Michael 
We have a Thomas Charles. But I love Thomas Joseph then you could have TJ for short.
I love your taste in names!
I have this thing that middle names should honor people, so what about…
Thomas Victor, in honor of his wonderful mom?
Otherwise, just to fit with your style…
Thomas William
Thomas Henry
Thomas Andrew
Thomas Drew
Thomas Nathaniel
Thomas Elliott
Thomas Everett
Thomas Emmett
Thomas Graeme
Thomas Alastair
My final recommendation is very quirky, but I just love it. It’s the name my friend almost gave to her son – I was all disappointed when she didn’t:
Thomas Owl
Thomas Henry (nick name for Henry could be Hank)
Thomas James (TJ or he might go by his middle name - James, Jim, Jimmy)
Thomas George
Thomas Trenton
Thomas Justin
Why not look in your family tree and see what you can use there? Yes, I realize that Justin is your husband’s name, however, it’s an idea.
Why not name him or her after grandparents? Or even great grand parents? Mix it up - try Thomas as the middle name.
Robert Thomas
Thomas Gregory
Charles Thomas
Christopher Thomas
Thomas Edward
Richard Thomas
There are a lot of great names out there. Go to your local day care and find out what names are being used in the infant room - this will help you know what NOT to name your child! Especially as you get closer to the deliver date…
My first thought when I read your question was “James” then I saw that your oldest has this middle name so I don’t know how you feel about that.
I like Matthew James or Matthew Charles. To go with Thomas maybe Matthew or Michael.
addedWhat about Justin Bradley? Not a third but still honoring them and you could called him JB.
I’m not great with this sort of thing, but I will say to always look at the last name too, and make sure the 3 initials don’t spell anything you wouldn’t want on a monogram or that they don’t give other kids something to tease your son about. Friends of mine have a “F.A.D.” and others have "A.M.P.) which caused the kids to be teased in science class when they learned about amps and wattage! And those are the tame ones. I can’t even list the worst ones I’ve seen.
You could also think about whether you want to name him after someone in the family, either with the exact name or with the first initial.
Definitely Thomas Patrick😊
Thomas William
Thomas James
Thomas Bradley
Thomas Michael
Thomas John
Or Thomas as a middle name
William Thomas
John Thomas
Grant Thomas
Thomas Michael
Michael Thomas
Thomas Andrew
Thomas Alexander
Thomas Benjamin
David Thomas
Thomas Bradly
To me, I like Justin more for a first name: Justin Thomas ( you might get JT then, be aware!)
Love Elizabeth Jane for a girl!
Good luck!!!
My first thought was Thomas James, with TJ as a nickname, until I saw that you already had a James.
Thomas Edward (Ted or Teddy)
Thomas Brady
Thomas Allen
Thomas Gavin
Thomas Dylan
Thomas Francis
Thomas Lee
Thomas Michael
Thomas John
Thomas Ryan
Jefferson.
Thomas Jefferson is a truly awesome name.
NOT Edison. Thomas Edison was a jerk.
I like the sound of Thomas Bradley. It has a nice ring to it and ties him to your husband. My ex-boyfriend’s name was Thomas Edward.
Thomas Edward
You can use the same middle name if you wanted to.
All the girls on both my mom & dad’s sides of the family all have the same middle name.
Kind of an odd coincidence that it was a family name on both sides.
“The Train”–It rhymes with the other middle names (James and Jane), and would so rock with whatever your last name is. This kid’s gonna be fun.
Thomas Paul
I think it would be neat to keep with a name that has an “L” - for obvious reasons.
I hope you nickname him Thomas The Tank!!!