It may take him several days to get used to the space. Lots of hidey places and plants help so he doesnt feel exposed. Also, sometimes bettas don't like so too much space, they vary from fish to fish.
You can do two small heaters in each corner, I've heard of that working, but for a 10g I'd simply put the heater in one of teh corners near the filter and the filter will circulate the warm water everywhere.
Some plants along the divider is alright, it's okay fi the fish see eachother once in a while, sicne flaring is a natural thing for them.