Becky,
I can only tell you what I do, not what is correct for anyone else. It sounds like a cop-out to say "do what feels right" or "follow your intuiton" but it does seem to come down to that. I do three head positions for the front of a person: over the eyes, at the temples, and over the jaws. Some people will put their hands under a person's head and do the occipital ridge where the skull meets the neck. I sometimes do but not always. Then I go to the throat/collarbone area but I am careful not to cover the throat (voicebox) in case someone has an issue with communicating verbally (I don't want to further restrict their ability to speak their truth - maybe I couldn't anyway, but just in case!). I then do the heart area. As a male practitioner, I will place my hands over another male's heart. For females, I place my hands above the heart or go below the breastbone on both sides. Then to the upper belly, then to the lower belly, then to the upper abdomen, hands off and above the lower abdomen, then the legs, knees, shins, and feet from the top. I spend a lot of time at the feet as I usually feel drawn to do so. I think it might help my "psychic flow" to do so. For the back, I do two positions for the head: one over the top of the head and temples, and one over the occipital ridge and upper neck. Then I do the neck, the upper shoulders, the shoulder blades (great for pinched nerves), the upper back, the spine all the way down, the kidney areas (great for the adrenals which regulate response to stress), the tailbone, the backs of the thighs, the backs of the knees, the calves, the ankles, and then the feet. I usually do a top to bottom aura brush and grounding swirls at the feet. It takes about an hour depending on my guidance and intuition.
The more you practice, the more you'll get a basic sequence you're comfortable with. Then your guidance and intuition should be helpful in making situational adjustments.
As far as your little boy is concerned, I believe you can work on him and it will be as effective - if not more so - as all the distance healing that might come his way. Nothing is more healing than a mother's love - Reiki or no Reiki. Regular medical care is also important, but I know you know that.
I wish you and your son the best. I will keep you both in my thoughts. I wish you and your family a great holiday!
Ken
