Hi, I'm so sorry that your son (and you) are dealing with this. My son was diagnosed just before turning 17 (he's 27 now... and doing well!).
My son's initial treatment was six weeks of exclusive elemental formula (no food, formula only). This replaced steroid treatment but it can be used together with steroids. The more 'elemental' the formula, the easier it is for the intestines to absorb nutrients (allows intestines to rest and heal). Polymeric formulas are the toughest to absorb, however, they are palatable The semi-elemental or elemental are much harder to tolerate from a taste perspective but easier for the intestines. As your son is already on steroids, it might be ok to just go with the polymeric (but please confirm all this with your doctor! I'm just a mom sharing her experience!). My son never used semi-elemental but I know there are many kids here who are able to drink the semi-elemental shakes. (My son used a naso-gastric tube to ingest his formula.)
I've read of people using BRAT diet when flaring (bananas, rice, applesauce, toast) but, given the very strict limitations, this can only be used for very short periods. But, perhaps, your son can try for a day or two along with the nutritional shakes - just to give him a rest if he's having a tough time with other foods.
While my son was on the formula only diet, he was allowed clear fluids which included broth, jello, etc. Again, these won't give you son much nutrition on their own but might tide him over (with the shakes) when he's feeling his worst.
Going forward, you do need to plan for a maintenance med. Yes, you/GI want the steroid to work to bring the flare down, however, you need a maintenance med afterward to stop another flare from coming. My son has been on remicade for approx. 8 years and has not flared since.
Just to give you some hope/comfort, things will settle! Since being diagnosed at 16/17, my son has graduated from high school and university, is now working, is living on his own, has travelled with friends, plays sports, has had surgeries related to injuries, not crohns, and has recovered as anyone else would have, has had his covid vaccines, etc. So, although it can take a bit of time and it can seem overwhelming at the beginning, once you find the right treatment, he will feel better and will go on with his life.