I leave our heating on low. It's normal for the bits that aren't in sleeping bag to be colder, but it's not that cold atm he should be stone cold. Does the room feel cold to you? That's your best gauge. The recommended temp is between 16 to 20, but the 16 end always felt too cold to me.
Can't you get one of those thermometers? We have a gro egg. I admit we do keep it a little warmer in his room (18-20 rather than 16-20) but it is reassuring to see how cold or warm it is and know it's fine.