Hi My baby is 5 weeks old If we make him sleep on his back, he never sleeps. He grunts loud and does something similar to choke and vomits in sometime . Believe that’s called reflex, We tried him in a big cot, and in a little bed, or our own bed- any flat surface on back he feels very uncomfortable Any idea or suggestion to make him sleep on back and why this problem occurs ???

Red Nose recommends that the safest place for baby to sleep is in a cot that meets the mandatory standards, on their back on a firm & flat mattress.
Research has shown that babies manage their airway most efficiently in this sleep position & environment which reduces the risk of Sudden Unexpected Death in infancy (SUDI)
https://rednose.org.au/news/why-should-you-sleep-your-baby-on-their-back

What position does he sleep in?
Have you spoken with your Child Health Nurse or Doctor about this?
Has reflux been diagnosed?

Red Nose has the following information about Gastro-oesaphageal reflux (GOR)
Red Nose recommend that all babies, including those with GOR, sleep on the back on a firm, clean and well-fitting mattress that is flat (not tilted or elevated) to reduce the risk of SUDI.
Regurgitation in infants is normal for up to 50% of babies less than three months of age and 70% of completely healthy infants under twelve months of age. Most of this regurgitation resolves spontaneously after 6 months, and completely by twelve months in 95% of babies.

Babies with GOR should be placed to sleep on their back from birth on a firm, flat mattress that is not elevated.
https://rednose.org.au/article/my-baby-has-reflux