What kind of LLC pays tax?  Most of them are partnerships or S corps, so it's really the owners' taxes you are estimating and you need to know what else is going on with their finances.  If it's paying tax as a C corp, tell him that when he gives you the profit you'll multiply it by 21% and bill him $100. 

Or is he talking about quarterly payroll returns?  There are companies that do that, even for small business, better and cheaper than you.