It would seem to me that a misunderstanding of who or what the God of Abraham, Issac and Jacob is would be less offensive than a denial of the entire concept of God. Jews seem to be instantly disfellowshipped for speaking out in favor of Jesus of Nazareth, yet how many Jews have been disfellowshipped for speaking out against the whole idea of God?
What he has done is more than simply lapse into agnosticism. He has actively embraced a different religion. Now, the fact that he and you may think that his beliefs are the "fulfillment" of Judaism is irrelevant. He publicly professes beliefs which are anathema to Judaism.
Jews seem to be instantly disfellowshipped for speaking out in favor of Jesus of Nazareth, yet how many Jews have been disfellowshipped for speaking out against the whole idea of God?
I've already pointed out the distinction between an agnostic or atheist and a person who has converted to another religion. I've also said that a person who struggles privately with disbelief would be treated differently than one who actively speaks against theism.
Why don't we start with this:
"You shall not make for yourself a graven image, or any likeness of anything that is in heaven above, or that is in the earth beneath, or that is in the water under the earth; you shall not bow down to them or serve them...."