things I have looked for when employing people to work with my dds (dd1 is severe ASD, dd2 has traits; they are now 7 and 4):
patience. most people think it is a given, but I really do mean a different level of patience. you need to be able to be patient when a child ha repeated the same thing for hours (potentially) - not just not getting irritated, btu respoding in any way might reinforce the behaviour unintentionally. equally, when faced with reading the same book, yet again, then be aware of not giving away the fact that you are stifling multiple yawns - this book (puzzle/game/toy) is the most important thin to that child at this point in time - it is their opinion that counts, not yours! of course, having said that, no child can do exactly what they want all the itme, so diversion and distraction away form obsessions needs to happen too, but there has to b a balance.
will your dh be doing any out of hours help too? since it is a residential school, this might be an option. so the above might apply more thanit woudl in regular school hours, iyswim?
on any trips out into the community, your dh needs to have a thick skin. he needs to not mind funny walks/noises/reactions, andhe needs ot not mind the public's reaction to all of those. he needs to be hyper aware of the children's actions and reactions (and possible actions and reactions), and be thinking 2 steps ahead all the time. the same goes for time in the classroom - anticipation is often the key: think through any potential problems, and work through what the solutions might be. knowledge and preparation are power.
woudl your dh be working as TA to one particular child, or as a general class TA?