Hi, We live on Sunshine Coast QLD and it is already getting very hot. We have a two storey house and babies room gets the morning sun which means his room has hit 28 degrees already. I have started keeping his blinds closed in the morning and opening other windows for cooler airflow in the morning and then his window gets opened in the afternoon. He is still on two naps a day. Generally his room sits at 25 degrees during the day and I have window and door closed with ceiling fan running. The room Is becoming increasingly stuffy. Should I be leaving his door open at this temperature? Should we install an air conditioner? The sun on the window in the morning is too hot to have it open.

Red Nose does not recommend a specific room temperature where baby sleeps, but rather dress baby according to temperature.
It is not necessary to monitor the room temperature or to leave the heating or cooling on all night, as long as the baby is dressed appropriately for the room temperature.
https://rednose.org.au/article/protecting-baby-from-overheating-during-sleep

It is ok to use a fan, which may help to circulate the air.
If his room remains cooler & less stuffy, then leave the door open.

Red Nose also has this explanation about use of fans.
With regards to the use of a fan in the baby’s bedroom, a case controlled study of 185 SIDS babies and 312 controls found that the use of a fan was associated with a 72% reduction in SIDS risk.4 The reduction in risk was more pronounced in adverse sleeping environments. For example, when room temperatures were greater than 21°C, there was a 94% decreased risk of SIDS compared to no fan use. Fan use was also associated with reduced SIDS risk at lower room temperatures less than 21°C. The mechanism for this protective effect is unknown, but the authors suggested that fan use may reduce rebreathing of exhaled carbon dioxide trapped near a baby’s airway by bedding.
https://rednose.org.au/article/room-temperature