Sorry mate, but I don't like this at all. There is no engagement there at all, he is shuffling around on a small circle, getting hit in the mouth by the side reins every stride. His back is not engaged, it is as solid as an ironing board and there is no bend whatsoever through his body. It is very 10 year old child making their pony 'pretty for pony club'.
Watching the video, I have an overwhelming desire to reach through the screen, undo those side reins and drive him forward. At the moment, if he were going any slower he'd be going backwards. There is no benefit in this lunging at all.
As for your trainer, is this the same trainer who told you to bump your leg every stride to increase reaction to the driving aids? I'd take what they say with a pinch of salt. Sorry.
Cinny needs to be sent forwards. Yes, some form of contact with the bridle is preferable than nothing, but far too tight side reins are not working for him.
As Anebel suggested, a German bungy type set up would be more beneficial to this horse at this point in time. He is currently being punished every time he tries to reach for contact and by continuing, you will train him to back off the bit and sit behind the vertical no matter how forward you have him.
Get him out on a big circle, at least 20m, and drive him. He NEEDS to go forward. He cannot release his back until he has an opportunity to go forward.