I would probably use a grid to get him thinking.....at first use the bounce poles but then maybe add in small cavaletti's, so he actually has to think about lifting his feet up more. He may just be taking the Bounce Poles for granted. :)
I would say grids too. Try asking your horse to lengthen and then collect to subtract and add a stride. Should make him more adjustable the more you do it. Good luck!
In the arena practise shortening and lengthining, ask for a fast canter down the long side and then sit up, raise your hands and turn a 5-10 meter circle, once out of the circle as for a forward canter again and then keep collectining and circling. It's hard to get Ottb to collect at the beginning, this way ^ you are adjusting the stride which is a start! They have to collect to make the right circle :)