Is it safe to sleep a baby in a pram? We are finding that this is the only place he will sleep soundly without screaming. What are the negatives of pram sleeping?

Prams are not designed as a safe sleep environment - especially unsupervised sleep.
Red Nose has the following information:
Never leave your baby unattended in a pram or stroller. It is not a substitute for a cot.
https://rednose.org.au/article/prams-and-strollers

Product Safety Australia has the following information about prams.
Avoid sleeping children in prams or strollers. Never have an unharnessed child asleep in a pram or stroller as they can move about and may be at risk of falling or entrapment. If they are asleep, maintain regular supervision.
https://www.productsafety.gov.au/products/babies-kids/kids-equipment/prams-strollers

There can also be risks of baby becoming entangled in a harness that is not being used, & risks of falls.