Albert Street and Queen Street have several clothing stores. There is one nice men's store on Albert street, near the Scotia Bank on the same side of the street. I can't remember the name of the store but it starts with an M and it is only a mens store.
They have nicer shirts and pants there.
Also Brodies upstairs has some clothes.
Watch the NIKE and brand name running shoes in Belize in the smaller shops. Most are cheap knock offs but the owners will tell you they are real. I got burnt with a pair of Nike the owner assured me they were real and they looked pretty authentic but fell apart after they got wet a few times.
If you want a good place to shop for clothes and shoes, Chetumal is the best place.
Down the street from Brodies on Albert Street, just a little way, on the opposite side of the street is a little indoor shopping mall where Romacs used to be. There are all kinds of little shops in there, you might find men's clothing.
Try a place called Hook up next to Abraham's Hair Salon in West Landiver area or Brodies on the northern highway it has a good selection of mens clothing probably similiar to the store in town I would imagine.