
Liz,
Just thinking aloud take and use what you like.
1 Try steadying him before he goes over the apex.
2 Once he is over on lead, keep pressure downwards on lead combined with treat near top of downside.
3 This is the one I would go with. Start again on a lower A-frame and teach him to run up and over without stopping.
4 Try to keep his head low, although Dobes don't bend that way naturally, if he sort of slithers over the top he will find it harder to jump off.
I must admit in all the time I was involved in training agility others as well as my own dogs can't remember coming across this. Mine would more often come to just above contact and jump off.
How old is he again Liz, I thought he was under a year, if he is I would be careful of doing too much agility, you can do control, tunnels, even dog walk but I would wait to start A frame, weaves and jumps.
Hope some of this helps
Anne