The room should be 18 deg then a vest a baby grow a sheet and blanket sounds right or you could use a grobag and they come with a room thermometer that tells you how many layers they should have for the room temp. Its just a guide as we are all different but to feel his temp feel his chest if he feels too hot or cold then adjust layers. His hands/feet may be cold which is common in newborns.
I remember stressing about this so much first time round!!! Sounds like he will be warm enough, but feel his chest to check, not hands as they will be cold and not a good marker for core body temp. FYI Grobag do little nursery thermometers and a guide which tells you how many layers to put them in depending on temp of nursery...might be on their website too...will check
tuff when my ds1 was a newborn I was paranoid about this and yes the grobag thermometer and guide to layers are great. I rememvpber when ds1 was a few days old taking him for a walk in his pram, it was summer (Aug born) and I must have checked his temp and adjusted his layers a hundred times on a ten min walk
Feel the back of his neck, that's a good place to feel temperature.
My 9 month old wears: long sleeve vest, long sleeve sleepsuit, and 2.5 tog sleeping bag. He did wear 2 sleepsuits (no vest) last week when it was very cold, and sometimes I add a cellular blanket or a knitted cardigan as well.