When my DD was an infant she was on .5 mg for a bit, then she came down to .35 where she stayed until she was about three. This was in addition to sodium chloride. EAch child is different, and I would feel comfortable as your Dr. is monitoring your son closely. 

My DD now is 12 and takes .4mg Florinef.  We salt her food also.  This gets harder to do as they get older. I wish I had a salt tablet to give her, so I know she’d be getting what she needs.