Increasing topline muscle

[Qru]

My extremely high-withered TB has very little muscle on his topline, and was wondering what effective exercises anyone has used to increase the muscle mass.
We train in flatwork 4 days a week and jumping 2 days. Should I also lunge him in a chambon or pessoa?
cheers,
Qru
x

 

[VioletStripe]

Plenty of working correctly from behind - transitions transitions transitions!, the lungeing will help, particularly if some is done with the Pessoa, hill work if possible, if not pole work also does wonders xx