Mt Sinai has some very good pediatric Crohn's specialists, so they should be on top of the latest info on meds.
The SCD diet seems to help some, but not all, patients. My daughter is 23 and using it - so far, since 2006, she has managed to avoid the more toxic meds by instead using diet, Entocort, and/or LDN. So far, she has been symptom-free with the help of the less toxic meds and diet. But she is still tinkering to find the optimal combination of meds and/or diet which will knock her CRP from 2 to less than .8 (the normal range).
Here's more info about the SCD diet: www.breakingtheviciouscycle.info It's possible the diet can help Pentasa do a better job. You could try it for a month, to see whether anything changes. Just a word of warning - read the book, "Breaking the Vicious Cycle" and start with the introductory version of the diet for the first few days. It's hard to follow - the hardest part is learning to shop, cook, in new ways. It's not a low carb diet - it allows fruits, vegies, meats, poultry, cheese, homemade yogurt, nuts, honey, so is basically a nutritious diet.
LDN means "low dose naltrexone." This med is still in clinical trials for Crohn's. The first trial showed some success, and the best part was no serious side effects. This is why my daughter tried it - it helped her taper off Entocort, boosted her immune system, and kept symptoms away. Her labs, though, weren't quite in the normal range for CRP, so she is experimenting with SCD, Pentasa, and/or LDN, to find a combo that works to eliminate symptoms and result in great labs, without resorting to the more toxic meds. (the LDN/Entocort combo did this, but Entocort shouldn't be used longer than a couple of years) Dr. Leo Galland in NYC can talk to you about LDN and prescribe it, if you are interested.
The enteral diet can work, but as soon as your son goes off it, his symptoms are likely to return. So if you are seeing blood, you can try the enteral for a few days and see if it helps, or try the introductory version of the SCD diet and see if it helps. These might be the fastest options. You could also try enteral, then transition to SCD. LDN takes about a month to help most patients that respond to it, but some see results in days, and others do not fully respond until three months' time.