Thank you for your answer ! :)
That's a good idea indeed, i will try this for my next sessions, i'm not sure if this will work, but i can still try :)
You could take a rope and tie around the hoof, then you take a whip or stick and tap on your horse's leg and pull on the rope to lift his leg. Keep doing it until he can bow without the rope taking his leg and you tapping on his leg to bow.