Waalaikumussalam,
I don't think it is permissible to discuss the nature of Allah.
Bukhari :: Book 4 :: Volume 54 :: Hadith 496
Narrated Abu Huraira:
Allah's Apostle said, "Satan comes to one of you and says, 'Who created so-and-so? 'till he says, 'Who has created your Lord?' So, when he inspires such a question, one should seek refuge with Allah and give up such thoughts."
As for Da'wah, you should tell him whatever Allah has told us about Himself, e.g. that He is the Only God, there is nothing like Him, etc. And if he persists in asking about the paradox, then you may either tell him that it is not permissible in Islam to delve into the nature of God, or you may refer him to read and ponder over the Quran while leaving this question aside until he may receive guidance from Allah, or you may answer him in any other way you think would be most appropriate.
And I think that while giving Da'wah, the priority for us who don't have much Ilm should be to first and foremost protect our own eeman, lest we should fall into Jahannam along with the Kafir we are giving Da'wah to, and guiding him to deen should be second to that.
Wassalam.
