I don't know to be honest, We put a fleece blanket over our quilt and if it gets really cold I have been know to put a jumper on myself (in bed). Not quite sure about how you would keep ds warmer. My dd (she is nearly 3) now has a toddler quilt with a small blanket on top but if she feels too warm I will take the blanket off.
Sorry if that is not much help. Do you have a baby monitor? the only reason I ask is that ours has a thermometer which tells us how warm the room is.
Fleece blanket on top of the mattress and underneath you works very well (do this every year for my two DC). If it gets really cold you can sleep on top of a duvet as well as underneath one , really keeps the warmth in.