The birth certificate says the mother is Emily King. I'm afraid I think Emily Lena King is a different person.
I can't find James Russell King's marriage certificate as I don't know who he was married to.
As both Valda and I have suggested, it is much more likely that the father was a James Russell (and she started to write James Russell King to make it look legitimate). I don't think you will find a James Russell King, although I wish you luck.
And I'm not sure you are right to reject Emily Lena King. It may not be the right person but it does give a good reason for her son ending up in Devon. Have you established that the family who adopted him are not related to Emily?
(I see that one published tree puts Lewis' birth in Northamptonshire!)
Richard