How are young children/infants diagnosed with Crohn's/colitis?

Our now 4 year old son had had digestive issues from near birth in age. He would grunt like crazy each morning before passing a breast milk stool, then after starting solids at five months began a very painful journey of constant constipation. I would have to remove his diaper for it to even come out. He would throw up at the toilet, push out his rectum, and take an hour, just to come back and push a little more out later. So heartbreaking, and this continued for nearly three years. NEVER has the diameter of his stool exceeded that of my pinkie finger. Is that normal? Finally, we went to a pediatric gastro. at the age of 3. They said he needed miralax until the problem resolved on it's own. After 1 year on this prescription, I am beginning to wonder...He is very active, eats exceptionally well (lots of fruits/veggies), and is a great size for his age (a little below the 50 percentile in height and weight). His paternal uncle , aunt, and first cousin have Crohn's. Could he have it, too?