How do you work out where the connection is?

[JBee]

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?