Check the twist of the leather in the fenders, as your new saddle may be putting different torque on your legs.
And also measure the length of each stirrup, just because they are in same holes, doesn't mean they are the same length. It could be one is shorter/longer which is causing you to carry more weight, or brace yourself off of that foot.
And make sure your socks and jeans and even underwear are not twisted too, as that can press on nerves and cause your feet to hurt.
Have someone video you too, from the back especially, as it may be you are sitting crooked. Also look at where the stirrups hang on his saddle, compared to yours. That will add to pain felt while riding.
You might also need to go up a size in saddle, if you can, as that would move you further back and relax legs more.
To see if it is the saddle, ride his a couple of times, making sure boots/jeans/socks are same as when experiencing pain in your saddle. If you aren't hurting? Then it is the saddle, and while it may fit horse? It doesn't fit you.
Some saddles too, have a narrower twist in them, which is better for us women, because of the way our pelvic bones are. A wider twist will put strain on back, legs, feet, shoulders, you name it, as it is pushing legs further apart at crotch.
Get out a sewing tape measure and measure each saddle in the same area, and see what you get.