The GI doctor is the expert on this topic of constipation and cf. Geneticists are not experts on cf, they are only experts on how people inherit genes and how to interpret genetic tests. Yes, constipation is directly related to cf. My nephew has dealt with constipation problems since he was an infant but was not born with MI. My son was born with MI, but has not had any constipation problems. They carry the most common cf genes - delta 508.
CF varies from person to person. Genes provide a guideline of what the symptoms of cf might be, but they really can't predict how an individual cf person's disease will progress. While this is hard try not to worry too much, only time will tell how cf progresses for him. Just enjoy your son!
Sharon, mom of Sophia, 5 and Jack, 3 both with cf, aunt to Joseph, 9 with cf