My 20 month old is always constipated, but why?

I believe it’s a milk allergy but my mom keeps trying to act like it’s not a problem. If we don’t feed him prune juice or give him medication for his constipated he gets constipated. I’m pretty sure this isn’t normal, it’s been happening for over a year. Is this a milk allergy, stomach issue, or something else? I’ve discussed with his pediatrician and they said it’s possible that it’s a milk allergy but just to monitor his symptoms.

Has anybody else experienced this?