Ask for priority and don't gave a quick answer. You could say "f

blah blah (to show its difficulty level), it may take me at least ? hours /days to figure out... As my current schedual have been reserved for B,C &D project,this one will can be finished around Tue. Does it work for you? If you feel this one is more important I could reprioritize the projects on my hand. But I need your help for that part." Then you make a new order/new schedual and agree check with him whether it works from his point of view.

This works for certain people but not all. Don't rush to some simple dates,try to provide more detail to give the boss a hint about your work load. If he understands and supports you, great. Otherwise I guess it's the tree famous words people talked a lot recently. Good luck, and assume managers are rational first.