The med line-up isn't too bad. I understand why your GI chose them.
I don't think anyone is real sure why Flagyl works, but sometimes it relieves the diarrhea and stomach pain.
Pentasa is like Asacol or sulfasalazine. But it's more targeted for Crohn's in the small intestine ileum. Its an "aminosalacyclate" (works like asprin) to treat inflammation but it more often is used to maintain remission instead of induce remission. In other words it probably won't knock out the flare, but it may help if you don't react against it.
6-MP is an "Immunomodulator" which means its targeting the immune response to the inflammation. Good backup planning on your GI's part. It sounds like he knows his stuff. 6-MP takes several weeks to get up to full speed and probably will be monitored to make sure it doesn't get into your system too fast. You proably won't see any results with this one until much later.
Entocort - This one I'm taking now, and from my experience, it is a "lightweight" steroid. But it does target active inflammation in your small intestine and the beginning of your large intestine. If they suspect Crohn's where it is and they haven't got a CT scan or X-ray, this is also probabaly a good choice to start with considering multiple medications you're taking, and Entocort really doesn't have as bad side effects as prednisone.
Entocort is working for me. I started a little more than 2 weeks ago, so it's taken 2 weeks, but it is reducing my flare. I notice a difference now. But it's subtle. The pain and the diarrhea didn't go away quickly. I still have some flare symptoms, but now after 2 weeks, my symptoms have not got worse, and I can manage life with the symptoms better. I haven't missed any time from work in two weeks, whereas before the Entocort, I was out at least 1-2 days in a week because the pain and pooping were so bad and getting progressively worse.
What I'm saying is, unfortunately, with the medication lineup you have, you're probably going to be in pain and in the crapper more than you like. And it won't seem like it's working because if it takes Entocort a couple weeks to start working in you, that's a long wait to be dealing with pain. The good news is, you probably won't go backwards and your level of pain and discomfort won't get worse if Entocort works for you. Also, through whatever clinical mechanism Flagyl reduces pain and diarrhea (is it anti-inflammatory or immune-suppressing?), it might work in the short term and you might get results within a few days. But Flagyl may not work for you - there's that chance.
It sounds like your GI is being both agressive (so many avenues in meduication) yet conservative - not prescribing the most or biggest medicine lineup and this stuff is targeted for where he suspects the disease is active. If things get worse, let him know immediately. If things don't seem to be getting much better, give it a week or two.
Hang in there.