I'm not sure staying another year is going to help him with the deficiencies that he has. He's not going to get much more mobile.
This offense we run is not well suited for him either. 1 of the 3 options in the RPO (QB Run) is basically a non-existent threat to the defense.
That said, hope he stays because he is a good college QB and it gives us another year to develop his successor.
Whether he likes it or not, putting it on film will play a part in his draft stock. He's not going to get faster or start breaking runs, but if he wants any kind of success in the NFL - even if that means sticking around as a legitimate backup - he'll have to show he can slide in and around the pocket.
If he leaves, and is fortunate enough to be selected in the 3rd round, he'll have to pick up that skill and show it off while being on the chopping block. That's best case scenario if he leaves early. If he stays, he has a year to put it on film, potentially jump into the 2nd round and more guaranteed money, while getting more leeway from an NFL team than a 3rd-5th rounder would. A lot of it depends on where he'd eventually go this year, obviously.
2nd round is a sound decision.
3rd round is questionable to leave but not terrible.
4th or 5th round is an outright bad decision.
What he's told before the draft is one thing. It's still a risk on his part.