My great uncle is his grandfather?

Question

So what does that make us?

Answer 1

break it down further:

This is hard to write.

My Great Uncle Jack is a brother to my grandfather Oma

Oma has my mom and my mom has me.

Great Uncle Jack has Jackie who therefore is a first cousin to my mom, and my 2nd cousin.  Jackie has a daughter Fay (whos grandfather is my Great Uncle Jack) who is my mom's 2nd cousin and therefore my 3rd cousin (or cousin 2 times removed)

you are 3rd cousins, or cousins 2 times removed.

Answer 2

If he is your great uncle by marriage, nothing. He is your second cousin's second cousin.

If the great uncle, the boy's grandfather and your grandfather/grandmother were all brothers/sisters, he is your second cousin.