I guess we all get worked piled and piled on top of us until we finally have to say stop.
When I am given another task I always ask when it is required by, consider my current load, and then say whether it is possible or not there and then. I guess it is just good 'contract review'.
This can be done very politely, so long as you can quantify your work load (even if it is a best guesstimate) he cannot argue, and will have to get more resource if he wants it any earlier else it just wont happen - dont be defensive or offensive, just state the facts.