English to Latin and back to English Again.

I don't see why you need both lux and lucis--is that light of light? You seem to be using more Latin words than is necessary--sentence structure is quite simple in Latin.
I don't know which word you used for darkness, but all I know is that darkness is "tenebra", which is normally used in its plural form, "tenebrae, tenebrarum".
"him" would be "eo" in the ablative form (the one you want)
I don't know how you worked out the negative of "is", but I know just using "non" should work--"est non".
Could you break it down so I can see where you got all those words?

Deus lucis est, in eo nihil obscurum might be one possible translation. But Lucifer could be right in suggesting that tenebra might fit better than obscura. Those free translators are the pits. They will generally translate a sentence word-for-word which is silly because different languages have different word orders and different grammars.

Just as "all" could be "omni", but it depends on context. I actually got "tenebra" from Final Fantasy VIII's "Liberi Fatali". Sounded right to me.