Instead of going out, which can be very nerve wracking, offer to do things at his house. IBSers are homebodies because that's where they feel comfortable. Going out has many problems, like finding a bathroom, maybe having to wait for it (although women are more likely than men to have to wait in line for a toilet). And long things, like movies, are really worrisome. I'm sure he's thinking, "What if I get sick in the middle of it? What if I have to crawl over a dozen people to get out of the row? What if I have to keep getting up? What if I need to leave and have to make her leave too? What if she resents wasting money on the movie?" So a lot of times IBSers will just not go out rather than take the chance of getting sick and ruining an experence.
So, lots of dates at his place is a good place to start. Bring over movies to watch or board games to play. Offer to bring over a dish or cook it there or get take-out. IBSers are reluctant to eat out just in case the food makes them sick and restaurants, especially mom-and-pop ones, will only have one toilet or one per sex, so that can cause problems or, at the very least, stress. So eating in at his place guarantees him that if he does get sick, he has his own bathroom to go in and his full stock of medicine at his disposal and if he needs to lay down on the couch, bed or floor, he can do so. If you've bene dating a while, he should be comfortable enough to go over to your place without worry as well. There's always a fear, though, of using a person's toilet because I've been in some houses with serious water flow problems. What if it won't flush? Make sure you have a plunger in sight and if you do have a low-flow toilet or low water pressure, you tell him the trick to making it flush before he goes in--hold down the handle, flush it twice, etc. There's nothing more embarrassing that pooping in someone else's toilet and then not being able to flush it! And then having to ask the host for a plunger or call her in for assistance. Talk about being mortified.
If he's feeling up to going out, parks or historical sites are a good place to go because they offer restroom facilities and you're just wandering around without a schedule, so it's not like cutting a movie or concert short if he gets sick. Also, you're not pinned into a seat. And when you guys do go out, offer to take seperate cars, especially if you go to the movies or a concert or ball game or something like that. Sometimes just knowing you can escape without disturbing someone else if you have to can ease the tension that makes IBS flare up. It's a vicious cycle: IBS makes you nervous and nerves makes the IBS worse.
The other thing you can do to be supportive is have a look around here and, if you know what type he is (diarrhea or constipation), print off or e-mail some of the advice posts to him. Not only are there some things he can try to feel better, he'll feel so relieved that you're cool with his IBS and understand and aren't grossed out, etc. A lot of IBSers feel very alone because "poo" is a dirty word (no pun intended) in our society and a lot of people make a big production of being sickened by the thought of someone else having a bowel movement. People who can't handle just a little talk about bodily functions are about as mature as a 13 year old.
Another nice gesture you can make, if he has diarrhea, is to carry some imodium and/or pepto bismol pills in your purse and some chewable gas pills if he hurts after eating or bloats or gets a bit queasy. Then if he seems to be getting sick (you'll know when he starts fidgeting and glancing around--he's needing to go and is looking for the bathroom), ask him if he needs some medicine and tell him what you have. I carry large amounts of ibuprofen in my purse for my fiance who has a bad back and knees, and, right now, a bad shoulder. When we go on camping trips, I take my mini pharmacy, which is a plastic shoebox full of some of every prescription and non-prescription medicine I have. If he gets sick or hurt, he comes to me and I start digging stuff out and telling him what each thing does and let him pick what he needs. I don't have a problem telling him "My guts are tore up today; I've been 4 times already" and he doesn't have a problem asking me "What have you got for diarrhea?"
I remember a teacher in one of my classes--I think it was in high school--explaining the difference between real love and idolatrous love. The later is when you pin all your hopes and expectations on a person and put them up on a pedastool so high they can't help but disappoint you; you not only have unreasonable expectations, but you want your love to be god-like and above human matters. Real love is when you can tolerate the idea of the love of your life, your soulmate, the person you want to marry, having to poop and pee like other human beings. LOL. I tell people here sometimes if your mate and friends can't tolerate knowing about your condition--not that you have to go into graphic detail, but you should at least be able to say "I'm constipated to where I hurt" or "I have diarrhea so bad I can't go out"--then you need to ditch them because they're not your friends and your mate doesn't love you, or at least doesn't love you with real love.
And if you know he's at home sick, shoot him an e-mail or IM and ask how he's feeling. It makes a person feel accepted when others don't shy away from his IBS. And e-mail and IMs are better than phone calls since he may not be able to answer or talk for long. You can always have meaningful converstation electronically. That's what my fiance and I had to do for 2 years when we were dating and lived 1.5 hours' drive apart and could only see each other on the weekend. Lots and lots of e-mails.