Is my beta fish sick?

My fish hasn't been acting right. He lately has been down in the corner or swims down there. I have a rock thing he used to swim through but doesn't and two plastic plants. Also he is in a tank with a filter and has a sore it looks like on his back. Do you think he is sick?

It sounds almost like parasitic sores, especially the behavior and lack of activity/response. Most tropical fish stores have medicines if this is the case, look up a local Pet/Tropical Fish store. They would have both the staff & medicines, this determining potential causes & remedies.