I dont want to be a neighsayer .... but I can only find refrences to staggers as a illness cause by lack a magnesium but them references say its a illness in cattle and sheep, no mention of horses. Am still unsure why the clue specifies 'He' but theres no other answer it can be with that pattern
Okay cheers, I now learnt that the horse illness staggers (rye grass poisoning) is diffrent illness to the cattle illness staggers (magnesium deficiency)..