Drone spraying hasn't really hit the big time, from what I hear, and likely won't... With "electric batteries" and "range anxiety", carrying much capacity (acres covered per load) means the weight gets to be too much to be very practical. Can't carry much of a load at all, but if your guy is willing to return to home base often enough to refill with spray and change out batteries, AND has invested in enough fully charged batteries in reserve to make it work, more power to him. FAA rules won't allow drones to carry anymore than so much weight, because then they'd be able to do too much damage with "fly-in bombs" too.
Kind of the same problem that electric cars run into... batteries only provide so much energy density/#, take a long time to "refuel", and eventually, that "gas tank" is gonna need to be replaced after so many charges (and faster charging reduces the battery life). Outside of rust, most petro fuel tanks will last a lifetime.