Ancestry is best as you can download your results and upload them to My Heritage for free. I have very different matches on both sites so it’s been really useful.
Rebecca’s advice is the way to go.
Test with Ancestry.
Download a copy of the DNA test results and upload the file to My Heritage, Family Tree DNA and My Heritage.
Depending upon where the mystery parents were from a DNA test with 23&Me may yield results (they are the second biggest) but you cannot upload to them you have to buy one of their tests.
Your Husband received half his DNA from each of his biological parents. From his Father he would receive his Father’s yDNA and from his Mother, her mtDNA. He would pass on the yDNA to your Son but not his copy of his Mother’s mtDNA. You passed on your mtDNA to your Son but he in turn will only pass on his Father’s yDNA. Hence at some point in the future your Son could also take a yDNA test, this only looks at DNA that is passed Male to Male to Male to Male etc. & it could give a lead as to the Biological Paternal line of you Son.
For now reading the book and reading up on using DNA will be quite a task in itself.
Once the test is analysed you will be presented with a list of DNA Matches, that is people who you share family DNA with and a value will be given such as 449 cM, the cM means centimorgan which is the unit of measurement.
The higher the cM value the closer the relationship.
I chose 449 cM as an example as that is the value you will be looking at for DNA matches with your Son’s DNA test. If either of your Husband’s biological parents had any other children they would be a Half Aunt or Uncle and the children of these would be a Half First Cousin to your Son and the 449 cM value is an average of the shared DNA between Half 1C’s.
With either your test or your Son’s then to get unknown close matches linked into your tree can require a waiting game so do be patient and stick at it, results will come.
Good luck in your quest.