Trying to get 5 month old use to crib. He falls sleep with little fussing on stomach but cries loudly on back. I know sids danger. What should I do?

Infant sleep. While placing babies on their backs for sleep has had a very beneficial effect on the incidence of sids, it doesn't work for every baby. All babies are different, have their own temperament and personality. Some will just refuse to lie on their backs. If you have an infant like this, just roll with his personality and let him sleep as nature dictates. The risk of sids is really quite low.
Not a podiatry? Direct this to your pediatrician.
Sleeping on stomach. With some infants, you just have to let them sleep on their stomach. If you have tried all the alternatives and your infant (and you) will only sleep on his stomach, I would just let him do it. (i did this for one of my own children.).