don't be scared of this. there is nothing sinister about it.
i know its weird, but i would explore it and consider it gift, a chance to get a knowledge of something very rare and that few get to experience. it also shows he is open and probably has some kind of intelligence and intuition to be able to access and make some sense of these thoughts and feelings.
he will soon forget all about it, so try to find out more - if reincarnation is real, it is these memories of another person that may well help to make your son who he will grow up to be.
there is a theory that unexplained natural abilities come from previous lives, such as someone who is a natural athlete or artist or musician, and picks these things up easily.
this theory also applies to unexplained fears, phobia and emotions.
i also read of a little boy who went on and on so much about his past life, giving names, dates, places etc that eventually his parents looked into it...suffice to say he is now friends with his previous 'wife and kids' and they accept him as such.
another one was a child directed the parents to search under the floor in an old barn in another town and they found something that he could not have known about.
many think the kids are making it up, or have been coached, and if the kids were about 8 or 9 i would perhaps be inclined to agree, but at 2,3,4 years old, these kids have not faltered under intense questioning.
i don't believe you could coach a child of that age and i don't believe they could even begin to make something like this up.