This should be a non-issue by now, but anyways guys... a quick sift through a bunch of books on Google Scholar had Atlas always listed as a Titan, a second generation god (along with the Titans, the Third generation being Zeus's click), or simply, a god who sided with the Titans along with one or two of his siblings. However since the mythology is often conflicting, I suppose if one made a good enough argument, it is still debatable seeing as Atlas was not directly one of Gaia and Uranus's 12 children who are Titans without contention. Atlas was the son of Iapetus of the first generation of Titans, as was Zeus the son of Cronus and Rheia, who were also of the first generation. However, for reasons unclear, Atlas is classified as a second generation Titan, whilst Zeus an Olympian or third generation, I'd guess this is simply to do with the side Atlas choose to fight on.
So no, Atlas cannot be a Titan if we restrict our definition of Titan to only those direct children of Gaia and Uranus. However if we extend our definition of Titan, to the grandchildren who fought on their side, then Atlas can be a Titan, albeit 'second generation'.