First thing, stop trying to convert him.
When he has birthday or something, give him a book on cosmology. Steven Weinberg, Brian Greene or Brian Cox are all good. If he understands integrals, then go straight for a university textbook – Barbara Ryden’s Introduction to cosmology is good. If he’s more into pretty pictures, go for Giles Sparrow or Michael Benson. An alternative approach would be to give him a book on philosophy, but cosmology books are easier, at least in my opinion
I’m completely serious.
From your description it looks like he has deconverted fairly recently. If so, he is now in a big emotional turmoil. Imagine that you suddenly discover that you have been lied to all your life – THAT is how he feels. Deconversion means several months of emotional rollercoaster, and attempting to selling him another religion will only make matters worse. You can also expect an excursion into militant atheism before he finally calms down.
Your friend now believes (note the word believes) that atheism is an answer to everything. It is not, but he has to figure it out himself. He is now a hardcore materialist and will reject any religious ideas a priori. The upside is that he should now become very interested in the Universe at large. And you can very easily find God in cosmology, it just takes several years, so basically, the sooner you put him on this path, the better.
Trying to feed him Christian apologetics will have the opposite effect – apologetic literature is written by believers who have zero understanding of the atheist mindset. (Not to mention that writing style is usually pretty bad anyway.)
Trust me on that, I’m speaking from experience.