> I don't think it is any allergy to food
Sometimes a food allergy can cause digestion problem symptoms that are similar to inflammatory bowel disease. IBD is formally diagnosed with either an endoscopy or an intestinal biopsy. If Harley was diagnosed in a different manner, it's possible that he actually has a food allergy or some other problem. It may be worth trying foods with no (or novel) grains and novel proteins, just to see whether that makes a difference in Harley's symptoms.
> but I don't know much about IBD.
A lot of cats with IBD and some other gastrointestinal issues have problems absorbing B-12 from their food, and a lack of B-12 can cause digestive issues (vicious circle). You may want to check with your vet about B-12 injections. They've given subcutaneously (like insulin), and you can use your insulin syringes to give them. Your vet should be able to sell you a bottle or you may be able to find it at your local feed store; my bottle cost about $12 from my vet and lasts for well over a year (and we were giving extremely high doses, so yours should last much longer). Most folks trying B-12 supplementation start with the TAMU protocol here
http://www.cvm.tamu.edu/gilab/research/cobalamin.shtml and then adjust it to their cat's needs.
There are two main types of inflammatory bowel disease. By far, the most common is the diarrheal form; less common is the constipating form. In addition to the steroids that your vet is prescribing and beside the possible food change and B-12 supplementation, you may want to try things to help with the common issues (if you aren't already or have temporarily discontinued them). For example, if Harley has the diarrheal form, you might try probiotics or yogurt; if he has the constipating form, you might try supplementing with Miralax+water or with Zantac.
If Harley has the diarrheal form of IBD, then you might also want to consider switching to a raw food diet, which can be very beneficial to cats with diarrheal IBD. Additionally, most of the raw food diets are low-carb, and you can get many types with with no (or novel) grains and novel proteins, to see if that helps with his issues. Raw food diet warning: a raw food diet is usually fine for 'normal' cats and can be extremely beneficial in cats with diarrhea problems. However it can be helpful or *extremely* contra-indicated in cats with constipation problems: in the 16 years Gwyn had constipation problems, the worst thing we *ever* did was try transitioning to a raw food diet for a month. If Harley has constipation problems and you decided to try a raw food diet, *please* monitor his bowel movements / straining / etc very carefully!
When we tried a raw food diet, we tried Nature's Variety, mostly because they had a trial buy-one-get-one-free coupon that you could sign up for here
http://www.naturesvariety.com/tryit . You'll need to know the name of the store you're getting it from though; if you decide to try it, you can use their online store locator to find a local store and then call the store to see what they have in stock.
Also, if Harley is prone to constipation issues, I would recommend bookmarking this article on the management of constipated cats
http://www.theveterinarian.com.au/clinicalreview/article757.asp. Table 2 was my bible for quite a while; it gives a selection of various drugs that can help with constipation in various ways. So if the constipation gets worse or the cat stops responding to one medication, you can look into supplementing or replacing the current drug regime with other that may complement or replace the current ones.
There's also a feline IBD YahooGroup. The old one, unfortunately, closed down; the new one is here
http://groups.yahoo.com/group/felineibd1. I'm not familiar with the new group, but the old group was mostly familiar with the diarrheal form; if Harley has the constipating form then, with each piece of advice they give, please check whether it's appropriate for the constipating form: although there are some things that can be helpful in treating both forms, quite obviously things that can help with diarrhea can be wildly inappropriate for treating constipation.