I have no idea who your GI is, but I like him/her already!
The doctor seems to be on the ball.
As to your question, large studies following thousands of patients over several years showed that:
1) Remicade plus imuran or 6mp was a little more effective than Remicade alone
2) Remicade without imuran or 6mp had somewhere around 10-15% of patients develop antibodies to Remicade each year they were on it. Remicade *with* imuran or 6mp dropped the odds of developing antibodies to Remicade to somewhere around 3-5%.
So it helps it work a little better and improves the patient's odds of being able to stay on it longer.
On the negative side, the combination had slightly higher rates of opportunistic infections (sinus infections. skin infections, lung infections, etc).
And there is a very small but serious risk of a certain type of cancer called HSTCL (hepatosplenic t-cell lymphoma) that can happen in young males (teens to 20s) taking both in combination. It's very rare but almost always fatal.
So for that reason, they usually put teenage boys on Remicade alone.
A few years ago, when Remicade was one of the only treatments available, that was a little risky because developing antibodies to it means you'd need to switch to another medication. But now, there are several medications available; so *if* he does develop antibodies at some point, then there are still several other options/medications available.