There's a good presentation on this site:
http://programs.rmei.com/CCFA139VL/
that explains the risks with medications used for IBD (like Imuran, Remicade, Methotrexate etc.). The risk is very small and the risk of
complications with IBD is much more likely.
In our case, my daughter was so miserable and sick that we went straight to biologics and adding MTX and later Imuran was a no-brainer. I haven't regretted it even once though of course, like any parent, I worry. She hasn't had any side effects with Imuran really, just some mild nausea.
If your son developed antibodies to Remicade, it's a good idea to add something like Imuran or MTX to prevent him from developing antibodies to Humira, particularly since there are only a few biologics.
I'll tag a few parents with kids on combinations of meds: My Little Penguin, Clash, Farmwife