He'll remember anything that won't interfere with future things (e.g. he won't recall knowing older!Albus or anything anyone said about his own future). So he'll know about DDD and the people he knows here, though it could be slightly vaguer to him at first.

I was wondering what to do with him, canon-wise after finding the Elder Wand (which originally I was planning to happen after June) so slipping him back works out for the best anyway. XD So I assume then we can play out their first meeting, etc, with a fresh start. (Except post-Ariana he'll likely continue to pester Albus through the comm, so from there canon may twist?)