At what age can I introduce a comforter toy to my baby for sleep? My boy is a couple of days off 5 months and is sleeping arms-free in a cot in the same room as me.

Red Nose does not recommend the use of a soft toy/comforter for babies under 7 months of age due to risk of suffocation & overheating, & has the following information.

Keep soft toys out of the sleeping environment for babies under seven months of age because they may cover the nose and mouth and interfere with breathing.
The risk posed by suffocation by the presence of soft objects in the baby’s sleeping environment outweighs any benefit to the baby from a soft toy. It is therefore advised not to place soft toys and other soft objects in the cot for babies under seven months of age.

Seven-month-old babies are more likely to explore objects in their sleeping environments than younger babies. Some babies over seven months of age may appreciate a small object such as a soft toy to provide comfort and connection (transitional object) during times of separation from their parent.

Soft toys should never be placed in the sleeping environment of an infant under seven months of age. Soft objects in the cot can be a suffocation risk. Physiological studies indicate that facial obstruction by soft bedding may lead to complete airway obstruction, and/or hyperthermia, and/or accidental suffocation by rebreathing
https://rednose.org.au/article/soft-toys-in-the-cot