Recently I started to learn Dream on - Aerosmith and I realised that my fingers don't stretch as much as they probably should. What sort of exercises would help to stretch my fingers more and play more comfortable?
What exactly are you having issues with? I can't see anything in the tab that should be giving you problems with stretching...
To be honest, keep trying to play the song would be your best bet. It's just stretching so just stretch, I guess.
The more you stretch the better you get at it, but don't do over do it. Good things come in time
Learn Siege Engine by Buckethead. It is one of the songs that made me stretch my fingers like never before, I believe it will help you.
Just be careful with stretching exercises, you can over do it and injure your arm. If you experience any pain at all, stop stretching straight away. Try some small stretches to start with, and dont over do it, then only work up if all is OK.