Sure, but is that when you want to introduce him? You could as easily say: The earliest we can introduce him would have been in season 4, in Nevrast or during the move to Gondolin. I'm more interested in your ideas than in the limitations.
I think it would be nice to introduce him some time before the Nirnaeth, to invest some emotion in him before he goes. But it's fine that we haven't seen him yet, since he probably won't be relevant for a while. I guess we could say he's around, but he should perhaps not be included in casting this season - or should he? (I'm beginning to think he could just be introduced in the Nirnaeth, that would probably be fine)