How do you work out where the connection is?
 

My half sister, her son and my son all have ancestry DNA results.

A man who was adopted but has no parents details has come up as a match possibly to a half aunt of my half sister and I.

I think it might be a generation or two further back but he's attached himself to our half aunt on his dna tree.

How do I work out how close a match he really is?

How much DNA does he share with each of you (or each of them), Julie?

Seems to be 189 cM across 8 segments for my half sister and my son

If he was a son of your half-aunt, he would be a half first cousin to you and your half-sister, and half first cousin once removed to your son. 189 cM would fit for half first cousin once removed, but if you google "how much DNA do you share with a half first cousin", the top result says 215-650 cMs. So it looks likely that you are right in thinking it's a generation further back.

Thanks Kate

I really get confused with this dna lark.

I will contact the person managing his dna results for him and ask what results they have matching with my half sister, her son and my son and see whether its the same as I got.