Your former dog is best where he is, and is the property of his new owners, who as your findings have seemingly confirmed are also good pet parents. That they do things perhaps a bit differently than you would if he were still yours is to be expected, but that is not cause for alarm or concern. Courts are filled with disputes over children by warring (divorced) parents--one claiming their ideas of raising the child being better than the other parent's, and only thing that does is harm the kid whom they claim to be speaking for. Nuts, isn't it?
In this case, he is their dog (you surrendered him to them, yes?), and their decisions qua owners must by you be respected. Please know I undertand and hear that you are in a better place now, and worried about him. Please don't beat yourself up about it. You did what you did because you wanted what was best for him, having since confirmed by the sounds of it that you did in fact make the right decision: you didn't dump him off or abandon him but, rather, found what sounds to be a good home where he is taken care of and loved. Having satisfied yourself of this (which means you care, and always did), it's time to forgive yourself and don't second-guess: you did what was best, in his best interests. Doesn't mean you need be a complete stranger in his life, should his owners wish to provide you updates from time to time, but that once again is up to them (and they are not going to be inclined to do so if you question or critique their pet-parenting, right?).
It's hard, I well understand, never an easy thing to give up a friend, but you didn't abandon him; by the sounds of it, you found him a good home such that it is OK to forgive yourself, knowing you did for him what was best.
All the best