I must have bought them all up on the last sale! I stocked up for 6months! LOL

Start at 2mg and see what happens. The optic neuritis is the thing I worry about for you... low B12 gives a similar optic symptom.
When severe, it leads to blindness.

If your son eats meat, and seems robust in other ways, no numbness in the feet, or severe fatigue things like that,
1mg a day should be enough to start. Once rebuilt up, most don't need the 5mg ...1mg is enough. But without testing you don't really know where you or he is. But if he uses heartburn drugs every day, he could well be getting low, as those block B12 absorption from food.

It is non toxic and inexpensive and so people can do it themselves. Not much else exists that is this easy and cheap.