I think it really depends on the weather Rails.
This summer I cant even use a grobag for Aidyn because its just too hot, so he just goes to bed in a singlet.
He has a 1.0 tog grobag that I am going to use for autumn/winter/spring and will just put extra layers of clothes on him during winter.
Do you have a thermometer in his room? Maybe that way you can keep track of how cold it is and whether you think he needs more layers. However you can usually tell if they are too hot as they will sweat and behind their neck gets clammy.