For most medication's , the first side effects listed are Nausea, diarrhea, vomiting.
I can't tell you if that is the medication causing his problems, or just the Crohn's disease. Most people start to have relief of their Crohn's symptoms while on the Remicade. I would call the doctor and let him know what's going on. What other meds is he on? You can check the treatment threads to see if anyone else has experienced this problem.
Welcome to the forum. I hope you get some answers soon.