First, you might not want to assume he hasnt called you back due to the wedding. Priests are very busy. I once didnt call call some family for a month. I knew there was no emergency and I was working late into the nights and not really taking any lunch. Any spare time went to simply rest and deal with the most urgent personal issues. Priests are even busier!
However even it is, you are looking at it only from your side. You forget that your decision to ignore your faith is a very serious issue for those who devoutly practice their faith. This is not say anyone should be rude : that would not be practicing their faith appropriately. But, as another poster already mentioned it is not an indelicate issue for a priest to address, particularly within his own family. I am sure he is struggling about what to do. Your non practicing family have put him in a difficult situation. Yes, it is always proper to love your family. However, the world wants you to believe that love is simply always condoning what the another person does. That is not true Christian love.
Abandoning your faith is a very serious issue for you personally, for your broader family and for the Body of Christ, which includes me and those of us posting here. But most importantly, your lack of faith is a VERY serious issue for you and those who have followed you. Your cousin the priest should love his family and anyone for that matter regardless of their behavior. However, giving support to a non Catholic wedding is not necessarily the most charitable act either. It could be very easy , I am sure, for him to just acknowledge and participate in the wedding. And I bet he wishes he could. But that would be a lie to God and to the rest of us and to you. It is very sad to him and to us that you do not have the faith you were raised in. It is always sad when any family member drifts away. We want you to share in the joy and happiness, and strength that knowing a loving God provides!
To simply acknowledge you sons wedding as just another event would be a lie to your cousin. The most loving thing he can do is try to find a way to let you know that your lack of faith is a very serious issue. How we communicate that sometimes is very hard to do. But do not think for a moment that his silence is only due to disrespect and lack of care. His silence could very well be from a very great desire to love you and your son. He may not have just yet figured out the way to communicate his familial love for you and also share the love of God with you. Simply attending your sons wedding as though it were a non issue is NOT true Christian love. God cares too much for you to let you think your family’s lack of faith is simply another option in vast array of lifestyle options. Your cousin an I long for you to know God and His infinite love! Saying nothing at this point may seem rude or inconsiderate, but it could also be a sign of great love! It could be a sign your cousin is struggling mightily to find a way to share his love for his family and at the same time be clear about the Gods love for you.
It is great that you supported his ordination. And you should have. But can you really expect your family in Christ to simply accept and support your falling away from the faith as though your agnosticism were just another hobby or sport you were involved in? That isnt to say there should be any lack of warmth, let alone hostility. But this wedding represents just another step your non practicing family is taking to reject the Body of Christ. It is another door being shut. Those of us with faith in Jesus Christ and His Father, including your cousin , are perhaps not communicating the most delicately and appropriately, but we are trying to get your attention!! GOD LOVES YOU , we love you and we want you to be in heaven with us for eternity! And it is so sad that you are, or any family member, is not in communion with Christs love for all of us!
Your sons wedding should be a sacrament. A wedding is not just another activity on Saturday like shopping or cutting the lawn. It is an opportunity to receive the grace of God! You and your son have decided to throw away that opportunity. It hurts me even. And I am sure its exponentially painful for your cousin. Dont judge him too harshly. If he is a good priest, he is not judging you and your son at all. He is only grieving at your loss. And I am sad for you too!
In any event, God bless you. May He find a way to penetrate your heart. May you find His loving hand touch you and your family at your sons wedding in a way that makes His love palpable and makes it incapable of living without. Im sure your cousin has these same prayers. And I will certainly be thinking and praying about you and your son and his wedding.