There is no need for shoes until they are walking outside. Clarks would love you to go and buy ‘pre-walker’ shoes, because they want your money, but barefoot is best, so I’d leave it until they need them for outdoors. Their feet grow quickly, so they’ll need a new pair before long.
What everyone else said. When he needs them for walking outside. The general advice is to wait for about 6 weeks, but please get them properly fitted. Clarks or any independent shoe shop that properly fits children's shoes. Don't buy supermarket shoes off the rack.