Firstly lots of hugs.
My dd2 (age 9yo) was born with her left arm missing from below the elbow.
I was lucky in that it was spotted at the scan, and so I had time to get used to it, and to mourn my perfect baby that I thought I had. It's a bleak time though. Let yourself have time to get used to it. I thought I was going to thump the next person who said to me "as long as it's healthy...".
While pregnant we made contact with Reach-there are similar groups certainly in Italy and France, Reach can put you in touch with them, but there are members from other countries. If you join Reach there's also a facebook group that is great for asking questions. Reach is better than limbloss, as limbloss is for arms and legs, and also adults, whereas Reach is children with arm loss.
Dd has proved that she is ever bit as capable as her big sister-the only thing she hasn't done is play the piano-she's learning the trumpet instead as you only need your right hand for this.
She does gym (and has won medals against "normal" children), little bit of tennis, dance, judo, swims, scoots (fast!), rides a bike (you can get a device that puts both brakes onto one hand), cuts with scissors, performs in panto, uses the computer and mouse without difficulties, ties her school tie herself, does shoe laces, can do buttons, zips, sewing...
I don't think there's anything in school or out she has not been able to join in with, although she has been known to try and use it to get out of stuff she doesn't want to do 
She currently uses a myoelectric arm, and a "cosmetic" (looks like a doll's arm) one for dance. She got her first arm at 5 months, and wore a cosmetic one until she was about 18 months, when she got her first functional one, followed by her first myoelectric at about 4yo.
Most children now don't use a prosthesis-we called her first one the world's most expensive teether, because all she did was pull it off to chew on it.
There are adults who come along to Reach gatherings who were Reach children, and they are very inspiring for them. There's a pilot, a doctor, paralympian swimmer, teachers including a games teacher, lots of other people who do "normal things" and they drive cars, play sport, cook, do all sorts of things that you'll be wondering if your ds will ever manage to do.
It took me about 6 months to really get over that "why her" feeling. We even managed a few smiles over things. I remember one time being stopped by a concerned old lady when dd was screaming her head off in the buggy.
"I think your baby's broken her arm"
"No" I said. "she's just hungry" and continued off at speed to where I could feed her.
When I arrived I saw that her prosthesis had twisted round to an impossible angle. No wonder the lady thought she'd broken her arm! Surprised she didn't contact the police. 
I'm not sure now whether dd2 would choose to have two hands or not. Sometimes she says she wishes she did. But at the same time, it gives her oportunities that she wouldn't otherwise have-and she loves being the centre of attention, and she has some very good friends that are the same as her. I would choose two hands for her every time, but she is (usually) comfortable with who she is and how she is.
I said to her when she started asking "why me" that all children are born with a little bit that isn't perfect, and hers is just more obvious in that you can see it. She loved that and has repeated it many times.
One adult member of Reach was born with no hands (above elbow). He is very much a get up and do-er. He does fundraising talks for Reach, which I believe include peeling and chopping an onion. I think he said the most popular question asked him was how he does his flies on his trousers.
Your ds will surprise you in what he can do. He will achieve what he wants to do, and you will, in time, be able to look at him and feel pride and joy without noticing his arm, for it will be just part of him.