Hi S.,
I'm not quite sure I followed exactly what has gone on however if your son is acting constipated and your "gut" tells you something isn't right then please follow up on it.
Watery or liquid stool can actually be fecal liquid moving past an obstruction. In general you should expect your son to have at least one bowel movement per day that is relatively soft at the vary least.
Our instructions were for a goal of two stools a day of "mashed potato" consistency. We ended up having to increase to 1.5 caps twice per day until that was established then back it down slowly to 1 cap in the morning and 0.5 cap in the evening plus a high fiber diet. Occasionally our boy will have a very liquid day but then things resolve on their own. We all have natural variation.
Because our son is also on another medication that can have constipation as a side effect we are not planning to wean him from the Miralx until the other med is complete. We hope to start weaning him from the other med sometime this year.
Every situation is different as is every child. If you are concerned you should make an appointment to have your son evaluated by a physician or nurse practitioner.
I have experience at both "waiting it out" thinking I was over reacting, and at having an evaluation that proved everything was perfectly normal. I'd rather go with the second one, it is easier on everyone and less likely to have the early stages of something missed.
K. H.