[Marilize]

If he is already staying, you just need to increase the distractions. It basically sounds like he knows what stay means, but he isn't used to staying when there are distractions around.
You can either lengthen his stays or increase the amount of distraction.
What is your release word? This is the most NB thing when it comes to stays.

You have to build up to the great stays. Cash recently finished another obedience course where the trainer would distract them with toys, other dogs and food. The distractions are gradually increased and only when he has mastered the previous distraction.

Also, when you treat him, don't let him come to you. He needs to learn that being away from you does not equal less or no treats. At the end of the stay, move towards him, give the release word when you have reached him and then treat. Don't ever let him move from the spot where he should stay - go to him. This tiny change made Cash incredible with stays.

The other big mistake that people make with stays is to keep the stay going until the dog shows signs of getting up (or he gets up). I am not saying that this is the case with you, I am just throwing it out there.
Every time he gets out of a stay, the process will be that much longer. You should end the stay, not him. So, I would actually suggest that you vary the amount of time that he is in a stay and quite suddenly give him the release word and then praise and treats. Doing this, you will condition him to listen to you, not act on a whim.

I hope this helps. I think it is a great thing that you are giving this baby a new lease on life. I am sure the training will go very well. Keep at it. Oh, and he is too cute! Love the colour!