Hi, my son is 18 months old and is sleeping on a single bed. We are currently just dressing the mattress with a fitted sheet, and he is wearing a sleeping bag. I have purchased a sleep suit and am waiting for it to arrive. When should I consider putting a flat sheet and doona on the bed?

Red Nose recommends not using pillows, doonas or other soft padded bedding until a child is no longer in a cot.
https://rednose.org.au/article/at-what-age-can-i-introduce-cot-bumpers-and-pillows
It is not uncommon for younger children to remove bedding like doonas, so consideration needs to be given to adding extra clothing to toddler if necessary.

I have also provided link to “Cot to bed” brochure FYI.
file:///C:/Users/lorra/Downloads/RN3356_Cot_Bed_DL_Oct2018_Online (2).pdf