So my mum was quite extreme possibly by mumsnet standards but from 5 years old she walked us round the house and talked about how to get out in an emergency.
Please teach your son in an emergency to pick up something weighty and hit the corners of a window and not the middle. The corners are the weakest part.
It could be with an ornament, a chair, a really thick book, anything that has weight.
Hit in the right place a window will break easily.
Teach him if he can't get the window to break, keep his mouth covered, and cover the gaps in the doors with whatever soft furnishings, clothing or bedding he can.
Also show him how the safety catches work if he isn't a windowsill climber by nature.
I don't think your ex should be leaving him alone, but fires do happen when everybody is in the house, and parents can still struggle to get to their children.
I have survived a house fire through arson as a child and so perhaps I feel quite passionately about it, but if you feel your son has the capacity, please consider doing some small age appropriate drills.