Hello, I am wondering when it is safe to start using blankets instead of a sleeping bag for a toddler/child? My son is still in his Stokke cot with all sides on. Thank you!

Blankets can be used right from birth, as long as they are light weight & tucked in at bottom & both sides of the cot up to chest so they do not come loose &/or cover head & face.
https://rednose.org.au/article/how-much-bedding-does-baby-need

Often once baby becomes mobile & rolling in cot, some often get out of/kick off any blankets which can place baby/toddler at risk of head/face covering. This is where sleeping bag become particularly helpful.                     
Is there a reason you no longer want to use a sleeping bag?
How old is your toddler? Is he making any attempt to climb from the cot?
Some toddlers will use large toys, loose bedding, pillows etc, stacked to help them climb from cot.
https://rednose.org.au/article/at-what-age-can-i-introduce-cot-bumpers-and-pillows

It is recommended to withhold any loose or soft/padded objects (eg quilts, pillows) till the child is over 2y & in a bed.