Lunging and stretching, also hill work
To me the best exercise to build up the top line is hill work with the horse's head low and a good walk to engage his hindlegs.
If you can't go outside, you can do a lot of simple exercise (circles, serpentines, figures of 8, shoulder-in, etc..) with the horse on the bit with his withers high and his hindlegs engaged.
An other way is to lunge the horse with a training aid (but not for long, 20-30 minutes maximum)
It takes time to build up the top line, be patient and you will soon see results!
Good luck :)
Hill work is by far my favorite.