Well, some kids just see things to comfort them. But then if he starts to see other things and gets scared by them and has names for them then you may want to take him in for an evaluation by a psychiatrist. At this point I would keep an eye on him and an open mind both ways, and hope it is just a little angel watching over him and hope for the best.
It could be part of a dream. It's not uncommon for people to hallucinate immediately after waking up because their brain is still half in dream mode.
It is said that younger children are able to see angels or ghosts as they have not been long left the spiritual world to be in the natural world. Smaller children have less of a fear of seeing such. As long as he isn't afraid then I would not worry about it and hopefully with time he won't mention it again if he starts experiencing frightening sightings then you may want to soeak with a doctor regarding it.
I think that angels, and ghosts generally show themselves in a way that is not aggressive and fearful to the person seeing them. That has been my case anyway. They are very gentle and come to me to prove that there is an afterlife and we are being watched over and loved. I think this is the case for your son. I look at my experiences, although not many in number, as a huge gift and a blessing. I would tell your son that he is being blessed and not worry about it at all, unless you have signs of aggressive behavior from these beings.
oops,it's great that you're posting for your little brother, he's lucky to have you.
Incidentally, it could be a sign of something impending, meant to support the child's spirituality, or it could be a sign that your little brother may be a psychic. Either way, this is a good thing. He's a lucky boy. I have always felt blessed since my contact with the other side began (20 years ago).