Talk to him about "technical debt" - use that term, because its an industry norm one that can be googled.
Basically what you have here is a mountain of technical debt - a poorly developed application which is now having to be maintained. Bad decisions early on in the projects lifecycle meant that short term views were taken for immediate gain, pushing difficulties in maintenance far down the road - and they have now landed at your door, not the original developers.
Technical debt is like owning a house. A house built out of cardboard, balsa wood and spackle. A house which is in a high humidity area. A house which was built on quicksand, and has no foundations.
Thats your house and you have been asked to re-wallpaper one of the rooms - sounds simple enough. Except that the actual wall, being made of balsa wood in a high humidity area probably has the consistency of toothpaste, and thus you now need to renovate the wall as well. But the structure supporting the wall can't support a heavier, newer structure, so you have to renovate that as well. And then the quicksand starts moving because of the new weight on it, and so you have to add in new foundations. Etc etc etc.
Technical debt is the prime reason why "simple" changes become major ones, but its a reason you can use to explain why expected deadlines are unreasonable in given situations - if your boss ultimately decides the extra time is worthwhile, and he puts in a reasonable deadline for the change, then you have at least won on that front.
If your boss still doesn't see the issue, then there is a larger problem and personally thats the point at which I would start backing away slowly...