This.
The chances are there is a DNA test involved somewhere.
As I say upthread people in the UK usually do them for one of two reasons: cos they are really into their family history or because there's a mystery they want to clear up. We still don't tend to do them purely 'for fun' which is more common elsewhere.
If the husband has done the DNA test he has done it because he knows there's likely children out there. But it doesn't have to be him. It should from the OPs comments that it probably isn't but it should not be immediately discounted as an option.
A match with one of his siblings, a niece or nephew or a known child would make it very easy to find him. Or even a first cousin if he's the only possible male member of the family who is the right age.
The concern here would be that the 2 year old, now 22 year old, possibly has a reason to do a DNA test. If his mother suspects that the OPs DH did a lot of shagging about she may have told them and this would give them a fairly decent reason to take a test; to see if there are more siblings. I would wonder if this was the most likely option.
The other thing that sticks in my head is he was aware of this last one but did abandon the mother in someway when the child was very young. Who is to say he hadn't done the same before and is aware of the existence of these other children.
The difference in that scenario is the OP herself. She knew he had a child when he met her. If he decided she was absolutely the one, then he may have realised that dumping the child this time wasn't a good look if he wanted to keep the OP...