Sheet protection might work. Select all the cells that he is allowed to
change and go to FormatCells and under the "Protection Tab" uncheck
'locked' and OK out. Then go to the ToolsProtectionProtect Sheet (you can
put a password in at this point but it's not necessary and probably
counterproductive in your case)

Now he will be able to change any unlocked cell but if he attempts to change
a locked cell he will get a message saying that the sheet is locked and must
be unlocked before changing the cell. To unlock the cell go to
ToolsProtectionUnprotect Sheet. It's still a bit messy cause you then have
to get him to remember to protect the sheet again after he is finished, but
you might be able to sort something out.

It's a handy tool, I sometimes have to protect sheets from myself cause I do
have a tendancy to get ahead of myself at times which can create the odd
minor dilemna.

HTH
Martin