This thread has really upset me which has compelled me to post.
Firstly, I'm very sorry for your struggles. It must seem like the rest of the world is lucky and you aren't, but it's not the case. I'm one of the 99% who you say have no issue having kids - and no, I don't have fertility problems. But my daughter died. Pretty unlucky. Sill devastating. But I still have a family I need to go on for, and so do you.
you do not know that you won't have another child which is so important to remember. You need to keep yourself healthy for your DS AND your future babies. Just as if you broke a leg or had a bad cough you would get help, so you need to for your emotional health.
Your son may want a sibling, but he doesn't need one. But he needs his mum. It doesn't matter if you think you aren't good enough, of course you are because YOU ARE HIS MUM. There isn't anything better for a 5 year old. You are the most important person to him and you just can't take that away from him.
Please, please get help. Please make a list of what will make Christmas lovely for your son, and then force yourself to be the person there with him, not mil.
And yes, there is something to be said for being grateful for the child you have, alongside desperately wanting another. It's possible to be both. Do be grateful for him, and show him the love you are telling us you have for him.
Try and take the good out of this thread, and believe the many, many posts telling you that you and your partner are all your son needs. You say he deserves more - yes, but it's YOU he deserves. Your presence, your smile, your cuddles. Make a gp appt, then go and get ds from school, get the Argos catalogue, and start writing his letter to Father Christmas. And put a happy face on it. You can do it. 
Please keep posting xxx