Hello, my son is 22 months old and very tall, we've attempted to move him into his own bed as we were worried about him falling out of cot. We placed single mattress in middle of room away from walls as per the advice on Rednose website but he keeps rolling out in his sleep and won't stay in bed, usually ending up screaming at his door. Could we consider using mattress bumpersunder the fitted sheet? Or is this a suffocation risk? Thank you

Has he been attempting to climb from his cot?
Some toddlers take a little while to accept being in a big bed.
Do you have the option of a toddler bed with a railing to prevent him falling out?
Have a look at out Cot to Bed information (partic around bed rails)
https://rednose.org.au/downloads/RN0407_22_Cot-to-Bed.pdf
“Portable bed rails:
If you have decided to attach portable bed rails to an adult height bed to prevent your child from falling, be mindful that children can become trapped if the rails are not fitted properly.
Before placing your child to bed check that the portable bed rails fit tightly against the side of the mattress and there are no gaps between the mattress and the bed rail.
This will help prevent a child’s body or head slipping through and becoming trapped.”

I’m not sure what you mean by “bumpers under the cot sheet” Are you referring to using pool noodles?
If so, toddlers seem to just go over the top of them.