He is 4 months and only wakes up twice? Wow! Most of them wake up every few hours to eat and get a diaper change. Sounds like he's doing well. I wouldn't have his room that hot but I also wouldn't ever, ever, put a space heater in a room I wasn't personally sleeping in. The hotter room can effect his breathing.
Why? I had one explode during the night one time. It literally went ZAP and bits of it flew all over on the carpet. I was laying on the couch asleep and hear the noise and instantly jumped up screaming fire. My husband grabbed the fire extinguisher as he came through the kitchen. We got it all out and I got new carpet as a result but still, it was the scariest thing!
So turn your thermostat up for the whole house. Close the vents in the rooms you don't want so warm.
Don't do electric blankets or anything like that either. My friend had an electric blanket for a while, it worked very well, one night her husband was kicking around in his sleep, woke her up, she sat up because it looked weird in the room and her feet were super hot. The electric blanket was on fire. They got out but lost their house.
Turn the heat up to 70 something at night and turn it down a bit, a degree or two is all that is needed, during the day. It will equal the bill out.