Inanna,
This thing he did 5 years ago - how bad? Are we talking defaulting on thousands of pounds or something?
It may just be that he doesn't have much credit activity to his name, and that blotch against his name is one of the more recent instances. What did you need a loan for anyway?
I had a run-in with HSBC when I was younger that scuppered my credit for a while, I couldn't get overdrafts, recieved bad credit card rates, couldn't take out loans, but it went away pretty quick the more I used what credit I did have to pay for things. I have a score of 999 now, excellent.
I would use something like
www.creditexpert.com - you need to pay to join, and then pay to see your report, but it won't come to more than a tenner, and you'll be able to see everything. This thing that happened, that he can't remember, might be something that can be resolved. Your report will give you the name of the claimant who placed the default against his file -- and he may be able to call them and reach an agreement to get it removed, or complain to them that they shouldn't have put that there in the first place.
Regarding general behaviour that will lift the credit rating -- If he ever goes over his overdraft limit, that needs to stop immediately. Overdrafts are considered credit, obviously, and receiving charges for going over limits may appear as defaults on his credit report / lower his score. If he has ANY credit cards, it helps to use them and keep the balance manageable, so that you can always pay at least the minimum payment -- or better yet, pay off the full balance once in a while. Call up companies and arrange all of his direct debits to fall shortly after his pay day, and encourage him to keep a close eye on his money.
Asking for loans or credit too often can have an adverse affect, if he's been knocked back now, I would give it at least a couple of months - maybe longer - before trying again. I'd think about why you really wanted one -- if it was to do with a home, I can understand the frustration, but if its for anything smaller, I'd be patient and either save up or work to improve the score over time..