I don't think Michael calling AJ dad takes anything away from Sonny or his place in Michael's heart. There's room enough for both.
That being said, I'm happy for Michael. I could care less about AJ but I'm glad Michael is able to have this time with his bio dad. If for no other reason than at least to know where he comes from because I do believe that's important.
I am very happy that he feels this way about AJ now. I'm glad they've gotten close enough that Michael views him as his FATHER! 3) I have no doubt he still views Sonny as his father as well, which is only natural..... but I'm over the moon that he was able to develop a relationship with AJ without being swayed by his "parents".