If the child was a legitimate son of Philip, you would think that the family would know.
So was he born before the marriage - seems unlikely unless Philip in England for some time, if you believe they look related.
Can you guess the age of the child from the photo - or is the photo undated so that does not help?
He would now be in his 70's, and so it would be best to try and find him sooner rather than later, just in case.
If you find out who she married we can start looking for wills and death notices and other related information.
Similarly, if we are sure of her birth we can look for siblings and information about them.
Have you looked for any of the family in trees on ancestry etc?