It's all very well to say 'FFS' but the poster hasn't read the new Rebellion vol.1 - he pretty clearly says he's read Boy Soldier in the Great War. That's a completely different edition.
BSitGW, sadly, doesn't have a direct sequel - it was the only book Titan published in that format, and likely to remain so now. It collects the same material as the Titan hardcovers 1-3, and the first few episodes of number 4 - so you'll want the Titan hardcover vol.4 and onward.
