Your husband needs a logbook. It could be paper, it could be electronic, but he needs a way to record billable time (and expenses, too, if applicable) WHEN IT HAPPENS.
No one's memory is good enough to be trusted for this kind of record. If he isn't making a record as billable events take place, then your records are no good. Which means either you're underbilling and leaving money on the table, or overbilling and risking all sorts of bad consequences. Since these are tax records, they need to be accurate enough to convince the IRS, too.
The good news is that this is a very common problem and all sorts of solutions exist, from a simple pocket calendar on up. I like TimeReporter, a Palm time and expense application. It is smart enough to track time by client, project, and activity, and lets you assign different billing rates for each. It will record time either as you go, with a built-in timer, or after the fact, and will import Palm calendar events. Syncs to a desktop application, which can create reports and invoices, dump to spreadsheets, etc.
Hope this helps,