I'm looking at this via my phone so take my view with a grain of salt. There are several options.
The jacket is certainly a frock coat that appears slightly too large.
I can't get the picture large enough to see if he is holding a hat or not.
If the frame is curved the appearance of bell bottoms could be an optical illusion caused by the bend at the bottom of the picture. Some soldiers were issued trousers that were too long and would roll the bottoms inside the trousers creating a similar look. It could also simply be a set of pantaloons. There were several units that received a mix of uniform items that don't seem to go together.
One Wisconsin unit ( forget the Regt) was raised as a Zouve unit but never received a full issue of Zouve clothing but did receive partial issue creating a mismatched appearance. The men proudly wore their crimson pantaloons until they wore out and went to great effort to replace them. I have seen a CDV taken at Vicksburg of a proud Sgt in a frock coat, undressed Hardee hat in hand and very clearly wearing pantaloons. I have seen a similar photo showing a soldier in a sack coat, Hardee & pantaloons. So a variety of uniform combinations are quite possible, especially early war. Though it must be remembered that odd uniform combos existed throughout the war.