Baby not crawling

by Tony Lewis
(Shrewsbury, Shropshire, UK)

My concern is about Baby not crawling. My little boy was born 11 weeks premature, but still can't crawl. He has now reached 15 months.
He does get around by dragging himself around the floor on his hands & elbows, but doesn't really move his legs. He can move from a sitting position to his hands & knees but then flops onto his tummy before moving.

Any help or advice?