That's a good point, especially since the Titus Mede we're shown is no fool. I don't understand either why he would want an heir that's LESS competent than he. Unless this is like the Buddha, who as Prince Gautama was completely sheltered from life's realities by his father. Attrebus isn't headed for the religious or philosophical life, though; he'll either end up as Emperor someday, or remain an adventurer.
Come to think of it, he doesn't have siblings, does he? If he's the only heir, than you'd think Titus would be trying to make him the best possible future emperor, not handicap him. Kind of a mystery.
Well, Titus won the throne by violence. Maybe he also wants Attrebus to believe he's some kind of mythic hero, because would a mythic hero kill his own father for power (actually, they would, did, and do. But Titus' prodigious acumen is more dedicated to battle and politics than mythology, it seems).
When Attrebus knows what to do, he has quite the silver tongue. If given more real life experience and training, he could shape up to be a pretty good emperor. In fact, being kidnapped and traveling with Sul is probably the best thing that's happened to him.
Like Sul said, he isn't inherently stupid. Just sheltered. From a literary standpoint, when a character learns he or she is absolutely nothing, they either languish and die or resolve to become something. Thankfully, Keyes had Attrebus go with the much more interesting choice B.
His royal signet ring, on the finger of a corpse.
Attrebus also has a birthmark on his side, which was "too convenient"ly charred over on the corpse Colin finds, which leads him to believe the corpse is false evidence to misguide him and throw him off the trail. Colin's a smart guy.
That he is, and I hope he gets more page-space in book 2, he seems to be the novel's "bad-ass normal" character. Actually, a lot of them have moments for it. Annaig is just a regular young woman from a random town, but she rises among the ranks in Umbriel. Glim shakes off his doldrums to learn as much as he can before trying to leave, And Attrebus has that awesome 'Just who [the hell] do you think I am?" moment. Sul kind of gets the bad-ass grandpa status because he starts off grizzled and powerful.