Honestly, him having a strong resistance to poison is not that much of a surprise, just based on his position in society, and his military side. It is not that uncommon for people in certain professions to build up a resistance to certain poisons due to regular contact with them.
As a note, the same basic idea is used medically in order to administer antibiotics to those who allergic to them. This procedure is used when the antibiotic needed is the only that will be effective against the pathogens the patient has. This procedure is simply accomplished by starting out administering extremely diluted amounts of the antibiotic (iv form), then slowly raising the concentration to therapeutic levels, it is possible to desensitize a patient to the antibiotic they are allergic to. Of course, they are being carefully monitored to ensure that they do not react adversely.
This pretty much the same procedure as gaining a resistance to a poison, such as the venom of a snake. By taking extremely tiny amounts of the venom at a time, then slowly increasing it over a period of time, on can become very resistant to the venom of several varieties. Of course, you can't always use this method, it depends on what the mechanisms the venom or poisonous substance uses. A poster below mentions cyanide and mercury. They were being sarcastic about it, but there is truth as well there in that those are substances that this would not work on due to how they work.