I could go two ways on this......
1) That coin does not look like it was "found". Not found anywhere but from within a coin collection, that is. So shiny and taken care of? My hunch is that it didn't belong to any 10 year old until he took it from a friend or family member, or the family member of a friend.
Especially since he's talking about other coins. Kids are parrots, he is repeating what he has heard. What 10 year old knows about Morgan dollars? Someone at his house or a friend's house is collecting coins, talking about them to someone (maybe him) and he knows where they are. Boom, he picks up a $1 coin and can make another dollar on it, because he is 10 years old and doesn't know the worth.
In this case, that coin is stolen, not bought. Would I take it? Well maybe. Read on.
2) Maybe the kid was the legitimate owner of the coin. Maybe he did "find" it fair and square. Once I found 3 or 4 half dollars while cleaning out a desk in an National Guard armory we were closing. They're currently in my coin drawer. Maybe a friend or family member gave him the coin as a gift and he decided he wanted to convert it into liquid currency to use for candy or whatever. He is a 10 year old kid after all. I've received coins for birthdays, XMas, etc, it's very possible the kid received it this way. If that's the case, he has the right to sell it for whatever he wants. If he asked $2 then you made him happy. This could explain the great condition of the coin.
If it were me, I would give the kid the benefit of the doubt and make the purchase. He's 10, if he doesn't sell that coin to you for $2 today, he'll sell it to somebody else tomorrow, or trade it for pokemon cards, or lose it. Could it be stolen from another coin collector? Yes, but don't we run that risk with anything bought second hand?