Hi there and welcome. Sorry to hear of your son's diagnosis but you have come to the right place for parental support and advice.
Historically, the way the ladder went with Crohn's treatments was 5 ASA's, then immunomodulators and then biologics. Biologics are the most expensive drug so insurance traditionally made patients try the 5 ASA's first and wait to fail before moving up the ladder. 5 ASA's are basically useless when it comes to Crohn's. There are some patients with very mild disease confined to the colon who do well with them but by virtue of the fact that your son got the disease young and has already needed a ten week course of steroids to get the disease under control, he would not be in the mild category.
Immunomodulators would be the next level. These are azathioprine and methotrexate. Most pediatric IBD docs will not use azathioprine for a teenage boy because of the risk of a rare but very serious form of cancer. So if you are going to try to work your way up methotrexate would be the next choice but honestly the immunomodulators are A) not as effective and B) have a worse risk and side effect profile than the biologics.
The most important thing is to get the disease under control quickly before it could do more damage. Research studies show that the sooner you could control disease the better shot you have at changing the disease course. Humira and Remicade have the best success rate. I have two daughters who have been on biologics for a combined total of 13 years and they have never had a problem with them. They have shared a bed with a cousin who came down with the flu, gone on mission trips where half the youth group came down with mono, have been exposed to strep and a multitude of other kiddie viruses and illnesses and they are the healthiest of all of their friends. One of my daughters had mild disease and did try methotrexate for a year and it didn't work so she moved on to Remicade and it works very well for her.
As far as Covid goes, there is a thread here that gives you all of the information for IBD patients. They are saying that the biologics do not increase your risk of getting Covid and there is some research going on that is finding that it actually may have some protective value should you contract the disease. The biggest risk factor for an IBD patient is actually prednisone so the GI's are suggesting that patients stay on their meds and not risk a flare that might have to be treated with prednisone and leave them more at risk. Also, the healthier you are overall the better able you will be to fight any illness that comes your way. Ironically, he was already on the riskiest IBD drug and at the most active Covid time.
I hope this answered your questions. If not, feel free to ask away.