9 month old not crawling

so yesterday, my son had his 9 month checkup. They have you fill out an evaluation form for things they do. He has zero desire to crawl. The doctor unofficially diagnosed him lazy which we totally agree with. my question is crawling and moving around something they just have to do on their own or can we push him towards it. right now I have him on his stomach, he's bawling. my plan is to leave him for awhile to see what he does. is this cruel? also, doctor gave a number for a physical therapist if we wanted to help him along but not sure if that's what he needs. just looking for suggestions. thanks!