For an old horse, the best is walks on fields with difference in height, it will be great for his toppling and also for his mental health!
To work well the top line, you need to worry about the abs (the abs make the back rise) so for that you can make shoulder-in or haunches-in in order to make him engage the hindquarters.
Then, you can work on go backwards, full extension of the neck or even bounce.
Finally, you can lounge him (on big circles in order not to damage his hock).
I hope i could help you :)