There's another dimension to this: The animation could be difficult to work with. It's not just that David is being directed to act more stoic, but the animation itself is also depicting Riku that way. His model is rarely very mobile, and not very energetic like his younger model was. This actually started way back in KH2, but back then we saw much less of Riku, so I think a lot of people simply didn't notice.
People have to remember that a VA in a dub is restricted by the movements of their characters on screen—both their body and lip movements. This isn't necessarily a problem in Japanese, because Japanese body language TENDS to be more reserved and stoic by custom, so a Japanese VA would have an easier time of inflecting accordingly in a way that sounds natural. But for English dialogue, Riku's body language is too rigid, and that puts real limitations on what an English VA can get away with in terms of energy or inflection on their end. It was the same problem with Terra, and with Aqua. Haley Joel Osment and Leonard Nimoy/Rutger Hauer/Christopher Lloyd can get away with much more because their characters are so much more visually animated. So, it's not David's fault, and it's not all the director's fault. It's mainly a disconnect between cultures on a very microscopic scale.
People have to remember that a VA in a dub is restricted by the movements of their characters on screen—both their body and lip movements. This isn't necessarily a problem in Japanese, because Japanese body language TENDS to be more reserved and stoic by custom, so a Japanese VA would have an easier time of inflecting accordingly in a way that sounds natural. But for English dialogue, Riku's body language is too rigid, and that puts real limitations on what an English VA can get away with in terms of energy or inflection on their end. It was the same problem with Terra, and with Aqua. Haley Joel Osment and Leonard Nimoy/Rutger Hauer/Christopher Lloyd can get away with much more because their characters are so much more visually animated. So, it's not David's fault, and it's not all the director's fault. It's mainly a disconnect between cultures on a very microscopic scale.