Sorry for the bump, but I was having the same trouble about a week ago and I found that you don't actually have to do all of expedition for him to appear, you only have to do the very first part where the natives assault the island. Kill them, then travel back to core city on your newly acquired jet ski and Aran will be in the bar in the residential district of core city. If the natives are too strong to fight head on, lay traps near the choke points and throw caltrops everywhere, then toss molotovs for area denial, since the AI generally will not path through fire if they're not already standing in it. Your allies should do 90% of the killing for you.
If you want Aran's equipment, use gas grenades to kill him which will not aggro him or anybody else, even if you do it right in front of them. He'll disarm them when they land, but just keep throwing them at his feet and he'll stand in the small cloud it creates and be contaminated to death, then you can loot his body freely for his amazing katana.
EDIT: Second part apparently no longer works with the most recent update, looks like they patched the gas grenade cheese.
