Two words and a potential third come to mind after reading your description. Let's start with:
fickle adj. changing frequently, especially as regards one's loyalties, interests, or affection.
If someone is fickle, it usually refers to their love life, as in they can't make up their mind about who to be with - not too keen to commit to a relationship. In your case, the person could be fickle in thought, meaning that they can't commit to a topic in conversation, constantly changing to something else.
The second word is:
a sudden and unaccountable change in mood or behavior.
This has a similar meaning to fickle but lends itself more toward their emotion and behavior. Joe's thoughts seemed to be driven by nothing more than caprice. The gist being that there seems to be no logical flow to his train of thought, constantly switching on a whim.
Lastly, we have:
caper v. skip or dance about in a lively or playful way.
While this doesn't directly describe the person in question, it does depict how their ideas are moving. He capered from topic to topic with seemingly no path to follow. In this example, it's less about non-commitment and more about free-flowing thoughts. Imagine him errantly skipping around like a child, landing on different ideas, seemingly at random, not staying on one for more than a second.