"Lord, You don't know? It's been nearly a 7 and half-mortal realm time, Later Devi Ganga adopted him and brought him here."
Surya dev could not fathom that there is a thing his lord does not know of but then thinking he might be in his Yog meditation it might be easy to not notice.
There are also lots of things his lord has to do and if he did not know something then there also has to be another reason behind it. He thinks that the reason is also the one that is not allowing him to see his son.
He knows how tricky Niyati is, It would not be strange if she decided to play her tricks on his son, After all a mother throwing her newborn in the raging river is not a normal thing, Even the Rakasha who are said to have no morals and such mother would never do such thing to her child.