Baby boots

Was going to order a pair of boots online for my baby. It gets really cold where I live so I want to obviously get him something good that will keep his feet warm. I decided on a pair of ugg boots comes in size 2/3 and 4/5. His foot measured 4 inches. Size 2/3 says 4.1 inches and 4/5 is 4.9 inches. I want them to last this winter which is only just starting. With his socks I’m thinking 2/3 may be a bit too small.. do I go up to size 4/5?

Opinions?

*** I should mention he’s 7 months old and not able to walk yet. Just want something really warm for his feet.