I'm going through exactly the same dillema. Thinking of making my next purchase an A/E and was wondering whether I could use my existing 15w Marshall.
What to do?
Check out what the knowledgeable folks on Chordie say of course!
I ended up typing in 'acoustic amp' into the forum search box and came up with a stack of useful information (sorry don't have an actual link for you).
To summarise what most posts were saying: You can use your son's Marshall and the sound will be 'OK' but a dedicated acoustic amp is going to bring out the richer acoustic tones far better. I acknowledge I'm speaking from a largely unqualified perspective here as I have yet to make that purchase, so hopefully some of those more in the know will offer you more qualified advice.
We are what we repeatedly do. Excellence then is not an act...it is a habit. Aristotle