He needs your help to know when the inside hind leg is coming forwards. To start with, try at a walk. Have him 'listen' (its a feel but you know what I mean...
You can tell him:' Now, now, now' as you see the leg coming up, so he learns what he is feeling/listening for. Then he tries it saying: ' now, now, now' as he feels the leg coming. Once you get it working at the walk, he can try it at the trot.
My first coach did this to me, and I have explained it to other beginners and they find it helps. One thing about dressage is perfecting your timing. Getting the feel of the inside leg lifting is critical to timing your aids for the canter transistion.
To ride well is the mark of a gentleman. To ride too well is the sign of a mis-spent youth. Athena the owl in "Outfoxed" by Rita Mae Brown.
-- Distrust any enterprises that require new clothes. EM Forster