My son is 6 months old and likes to fall asleep with something soft. When is he able to sleep with something to sooth him? He can roll, sit up, crawl

When he can stand. We lose about 100 kids a year in the US to suffocation in bed linen, bumpers, soft toys when they fall asleep on/in them. When a baby has the physical capability of getting away from these things with ease they can have them in their bed. Until then, it is a deadly choice. Don't offer it and he will adapt.
Training!! It depends on how you train him. There is no need for something soft, it is all a matter of habit. If you let him train you he will keep traing you. He may fuss for few days but once he realizes he has to fall asleep on his own, he will.