"Does this mean this person is related to me & sons father?"
Short answer, probably yes.
However, it may not be a simple relationship it might be quite convoluted.
"Now ancestry dna can tell if they are related from maternal or paternal side but some are unassigned"
How Ancestry does this is that they see who your different bits of DNA matches with and they already know a lot about which people are related to which other people.
So what they do is look for all your DNA that matches to one group of people and all the DNA that matches to a different group of people.
The DNA that matches one group of people is likely to all come from one of your parents and the DNA that matches the other group of people will have come from your other parent.
There can be problems though when you come from an area where there is a lot of intermarrying between the same few families as that can confuse things.
"...but some are unassigned"
I have an example like that. There was a woman who was in the USA who was listed as my 2nd-3rd cousin but no side of the family was given (it was "unassigned") as Ancestry couldn't work out which side of the family the link was on.
When my parents did their DNA tests it turned out that she was a close relation of my mum but a much more distant relation of my dad.
It turned out that we were half second cousins on my mums side (her grandmother and my grandmother were half sisters, same father but different mothers).
My great grandmother had died young (at 29) after having two children and my great grandfather then remarried and also had children with his second wife, one of whom was the grandmother of the woman in the USA related to me.
It turned out that my mum knew nothing about this and didn't realise that the person she thought of as being her granny was actually her step-grandmother.
Where my dad comes into it is that it turns out the second wife was very distantly related to my dad with a connection going back to people born in the 1690s. So this woman in the USA is also a 7th cousin once removed of my dad.
If you have several like that, that are unassigned then it might be worthwhile doing your own ancestry test as well. This will clear up a lot of those "unassigned"s.
It will also give you a lot more matches. Even if just one parent does it.
My mum has 20,000 matches on Ancestry, my dad has 18,000 and I have 16,000 matches.
Of those 16,000 matches, Ancestry says that about 9,000 are on my mum's side and 7,000 are on my dad's side.
So since my mum has 20,000 matches that means that I've gained an extra 11,000 matches just on her side (20k - 9k = 11k).
My parents have 38,000 matches between them so, in my case, the number of matches more than doubles by going back one generation.