I just feel like a really bad mom.

My son is 8 months and he’ll be 9 months on the 24th and he’s only 15lbs. Everyone goes on and on about how he’s small for an eight month old baby but my milk supply is low and i don’t know if that’s stopping him from growing. He won’t take formula, he rarely drinks breast milk from the bottle. On top of all that he won’t attempt to crawl. Whenever he’s on the floor he’ll reach and then just give up completely and just cry. My parents say to let him cry but i feel like it’s creating a bad experience and now he associates crawling with crying. He also won’t stand up on his legs anymore. If I put him on his feet he’ll automatically go to sit or start bending his knees. I just feel like he’s so behind and it all my fault.