If you press gently on the back of his ankle, just above the heel, his toes will uncurl. I used to sell kids shoes and that was how they taught us to fit kids. That said, soft soles are the way to go now. Anything with a hard sole is appropriate for once he's fully walking. Before that the sole must be soft to allow them to flex and feel the ground, to better their balance.

Soft soles. Like moccasins

Just wiggle the shoe slide to side while putting on and don't worry about the toes, he will straighten them out once they are on.