As it’s getting colder, my 8.5 month old isn’t sleeping through the night and will only sleep if I am cuddling him. I don’t co-sleep and don’t want to. His flannelette sheet is cool to touch so I have been looking at getting a wool underlay to put under his flannelette sheet however I am not sure if it is a SIDS risk. Any advice would be great!

Red Nose recommends that baby sleeps on a mattress that is firm, & flat (not elevated)
The major concern about a woolen underlay is its potential for creating a significant softness to the mattress.

A soft mattress or sleeping surface can increase the risk of sudden unexpected infant death if baby rolls over onto the tummy. Remove plastic packaging from the mattress and always make sure that the waterproof mattress protector is strong and a tight fit. Never put soft bedding under the bottom sheet, such as a sheepskin, as this makes the sleeping surface too soft.
https://rednose.org.au/article/what-is-a-safe-mattress

Are you concerned that your baby is cold?
You can always add an extra layer of clothing to baby for warmth.

A good way to check baby’s temperature is to feel the baby’s back or tummy, which should feel warm
https://rednose.org.au/article/protecting-baby-from-overheating-during-sleep