Hello Styx, 

Welcome to the forum. I'm very sorry to hear that your Mum has this diagnosis and that her health is now declining. 

We have some information on our website about talking to children that will hopefully help you with talking to your son. There is also a list of appropriate books but unfortunately the link is broken. I have flagged this and hopefully someone will get back to you in the next couple of days about that. 

In the meantime I do know that MacMillan Cancer Support about what to tell children when an adult has cancer which you can find by following the links here. 

In my experience, children are amazingly resiliant and although there will undoubtedly be tears ahead, he has you there to support and comfort him through this. 

Sending you our best wishes, 
Jenn
Cancer Chat moderator