Our baby is 7 months old and can move about independently and easily. At what age would it be safe for him to have a comforter in his cot and transition to using a blanket and pillow ?

Red Nose has the following information about soft toys/comforters in the cot.

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.
https://rednose.org.au/article/soft-toys-in-the-cot

Keep the cot free of large soft toys, pillows, bumpers, activity centres and anything else that could be ‘stacked’ to assist a young child/toddler to climb out of the cot.
https://rednose.org.au/article/can-my-baby-have-a-soft-toy

Ensure soft toy or comforter doesn’t have any parts that might come loose & risk being inhaled by baby. This includes toys that may contain batteries. Ensure they are not accessible to child.

Once babies are active & roll around the cot, they often kick blanket off or crawl out from under them, so is best not to use them. Sleeping bags (with arms out) are used by many families at this stage.
If you are concerned about your child being cold, you can add an extra layer of clothing.

Soft bedding is dangerous in a cot and should not be used. Soft bedding includes pillows, quilts, doonas, soft toys and bumpers. Soft bedding may cover the baby’s face and obstruct breathing and/or cause overheating.
Older babies in a cot can be at an increased risk of a sleeping accident by using pillows and bumpers as a step to climb up and fall out of the cot. It is safer to wait until the child starts to sleep in a bed before introducing a pillow or other soft bedding.
https://rednose.org.au/article/at-what-age-can-i-introduce-cot-bumpers-and-pillows

** Red Nose does not recommend placing a pillow where baby sleeps unobserved. It is safer to wait until the child starts to sleep in a bed before introducing a pillow. The Australian government states it is safer not to use a pillow at all for children under two

https://rednose.org.au/article/pillow-use