Masma, don't beat yourself up about it... which I know you'll be doing.
I have a similar story of my own, so I speak from experience.
You son has done good to owning up to it. Lots of addicts try and hide the fact they've relapsed.
I'm sure you know, you need to take each day at a time. He has the love of his family, and that's the best support he could ask for.
Stick with it and hang in there tight. You knew this was never going to be an easy journey, and you are in it for the long haul.
Don't do the whole sad face ( :-( ) because this is just a minor blip in your sons recovery. Don't be disheartened, and just be grateful that he's owned up and has "learned his lesson". It could have been ten times worse!
Your son will get there... as will you. Stay strong and good luck :-)