So a couple things - it sounds like they're telling you that he should be on a low-residue diet for right now. This is a good link:
http://www.ccfa.org/resources/diet-and-nutrition.html
They often suggest that when kids are diagnosed, until the Crohn's is controlled. It does not mean it's forever. The only restriction really is nuts, seeds and popcorn - otherwise it's very individual.
We have found through trial and error, that my daughter does better with seafood and chicken. Beef and other red meat (which she loves!) tends to cause diarrhea. She is also Lactose-intolerant, which can be common with IBD. She can still eat things with lactose, but avoids very cheesy things or things with a lot of butter. She also takes Lactaid.
Veggies (high fiber) tend to cause cramping and diarrhea for her. We do a lot of soups or very well-cooked veggies. She also does not do so well with high fat foods.
However, it REALLY varies. She has other GI conditions (Gastroparesis) so that limits her diet.
You'll have to figure out over time what works for your son. There are some kids who have pretty limited diets unfortunately. There are others who can eat absolutely everything!! We also saw a nutritionist at diagnosis who was really not the most helpful. She just said it was trial and error.
At the time, I was very frustrated - it would be easier if they gave us a plan!! But after reading kids' stories on here - it's really true. There are some kids who can eat any amount of junk food and be fine. Others don't do so well with junk food.
My daughter does not like juice (but never has - that's just a preference) but if he tolerates it, I wouldn't see any problem with OJ.
I would imagine MOST kids with Crohn's eat junk food every so often
. Mine definitely does!! She loves pizza, even though it bothers her stomach and will eat it occasionally. She loves dessert and chocolate.
The bottom line is you will figure it out. Unless you decide to do a specific diet, there are no hard and fast rules.
The only diet that has definitely been shown to work for Crohn's is enteral nutrition - typically 100% formula for 6-8 weeks. It induces remission (about as effective as steroids) but most kids need a maintenance medication like Remicade to stay in remission. Some kids drink the shakes and other kids use a feeding tube. Some (including my daughter) do supplemental EN to keep up their weight. There are many threads and EN studies on the parents forum, if you are interested.
Good luck!