How to dress 9 month old during winter

Feeling a little stress out on how to dress my 9 month old during our winter hols. How many layers is enough? 1st layer he will be wearing a cotton long sleeve shirt and leggings. 2nd layer knitted pullover and jeans. 3rd layer, winter puffer jacket, socks, boots, hat and mittens. Does he need a thermal or a heatech underneath it all? We will be in london during the christmas week and its our first time experiencing snow and winter as a family. Most days we be outside not indoors. Help a worried mommy out pls.