Sleeping with windows open?

I need some advice! My son is 6 weeks old and our bedroom is 20 degrees c, and I’ve read a baby should sleep in a room between 18-21 degrees c. I’m just worried he will overheat, he sleeps in a vest, a onesie and a blanket folded over, he seems alright and sleeps for 3 hours normally before waking up in the night, should I have a window open? Does he need to have a window open to get fresh air whilst sleeping? Help!!