AJC - you sound like you're doing a great job
Most babies will let you know if they are hot or cold and they'll cry to let you know. I would probably just have singlet, nappy and a cotton wrap snuggly around him. And the aircon not too cold..... comfortable 24 degrees perhaps?
Although i've read babies rooms can be 18 degrees and they are comfortable but that just seems too cold to me!!!?