If he cares for his kingdom's independence more than he cares for himself, there is an extra step he should take.
Early in the story, the royal family tree is explained. There was someone who had married into the royalty of another kingdom. This was a worry to me at the time --- this was someone who had a stronger claim on the throne than Slaine (because he's illegitimate) and an army.
So in this circumstance, he can declare this person his heir presumptive. Should he fall, the kingdom still has a monarch in exile (with an army.)
Of course, if he survives with a kingdom, he's kind of inviting them to assassinate him.