I've outfitted him with an arquebus and the class ability that gives him a bonus with same.  He is a back rank buffer/healer with (literally) a cannon!  He also has a second level spell, can't remember the name of it, that fires in a line with a rather decent area of effect.  It heals anyone on your team in the area and damages enemies.  That's been very usefull.  Granted, i'm only level 4 so i haven't delved into his midgame or lategame usefulness yet but he's not useless!