Why can't I summon my friend to help me?

I'm playing the original DS2 for Xbox 360 whereas he's playing Scholar of The First Sin

Yes, that's exactly why you can't coop (or invade).

Original DS 2 and Scholar are considered two different games, even with the DLC patch for original DS 2.

I guess the non compatibility issue comes from the fact that enemy placement are different between the two games.

Edit : seems like my answer is right for the PC version but I've stumble on different answer for the consoles version. Will continue to dig.