Unfortunately, whilst not a complete deal breaker it is likely to present significant hurdles. To hold a Class 1 medical (required for commercial operations) his condition will need to have been well controlled on 'minimal medication' for a minimum of 6 months (medications such as oral steroids are automatically disqualifying).
Even if his condition is well controlled he needs to be aware that any relapse in his condition will lead to lengthy interruptions of his ability to exercise the privileges of his license.
More information is available from the CAA if you search for 'caa inflammatory bowel disease'. (Sorry, I can't post links as a new user!)