Ironic, I wrote this advice about this exact same subject only yesterday, so save me repeating it, I'll link it.

Hi
My thoughts while reading this is WHY
Why does your partner want to tell him now
Why not wait til he is older

How is your son going to feel if you tell him and he is not ready .....
Does he see that his "dad" treats him different to his own biological child.....
Does he ever ask any questions

Think long and hard and get yourself and hubby councelling on the best way to tell him and make sure he is strong enough....
tell him and he may feel rejected by who he thinks is his dad.....
Myself I would wait til he is a little bit older unless he is sure about his relationship with not only his brother but also his "dad"
Luv Deb

I love what tracey said about this,,,,,,,,,,,

I think that if you do in the end tell him it should also b a gradual build up to this and also have him some support for him eg counceling etc,,,,,, he just may need it ,, it might not always be the best outcome either way,,, as you fell pregnent by some form of abuse it could very well upset your child .. he will need a great deal of support and love to get through this just like you have ,,,,i would seek professional advice first on how to handle this before i go tell my son anything

its great that your partner is dad to him ,,,,,

I'm sure you love your child and so does his stepfather but I will tell you something personal I grew up thinking my oldest brother was MY oldest brother it wasn't until I was about twelve that I realised he looked different than the rest of us. Now that we are all much older my brother is always asking my mum Who is my dad and what is his name now I know she is never going to tell him because she got pregnant when it was shameful holding hands let alone doing....... and getting pregnant and my mother married another man when she was 7 months pregnant and he gave my brother his name.To this day it's not so much that my brother want's to meet him it's just that he wants a name and why not everyone has a right to that as I feel it gives them some sort of connection .

I had a friend in a similar circumstance.  She met her partner (who is now her husband) when her son was only 2 1/5 years old.  He has grown up knowing her partner as being "his dad"  and although he is not the biological father, he is this boys "dad" He has raised him as his own.  His biological father was a drug addict and is currently serving time in jail for armed robbery.  My friend never wants her son to know his biological father, but did think he had the right to know who he was, and that the man who he calls dad, isn't his biological father.

She spent many years wondering how and when to tell her son.  She began when he was nine years old, telling him stories of how some kids have 2 dads. Their real dad who is the biological father and the dad they have now. The one who looks after them, buys them things, takes them places and has fun with them etc.

When my friends son was almost 12, he came to his mum and asked her if his "dad" was his real dad, and it was then that she sat him down and explained things to him.  He understood fine because she had introduced stories to for the past 2 years. She explained that he was in jail, but that if he really wanted to meet him, she would try to find out where he was and if he wanted to meet his son.  The boy, said No, he didn't need to meet him just yet  because he already had a great dad who he lived with. 

He is now almost 14 years old and still has no wants to meet his biological father, although this may change once he reaches adulthood and has his own family. He knows about his biological father and his full name, so if he does ever want to meet him, It's his choice.

Tracey