If you can, find another job. Your boss seems to have no idea about project management, resource management and software development.
In general, all you can give is estimates. Your estimates will get better with experience (for the most part, you'll estimate far more effort than you do today). And - as any sw developer knows, there's always an option of hitting some stupid issue which will take 90% or even more of actual effort (and hugely increase development time).
