Thanks everyone! He is worth his weight in gold, everybody loves having him around!
I have taken everyone's advice and this is what we did....
I have made sure to long line 2-3 times a week to reinforce his knowledge of the aids and voice etc.
Whilst long lining instead of doing plain circles I do lots of figures of 8's, cerpantines, transitons, turn on the forehand and even the disengaging techinque!
(Found his bad rein to be the right rein as he doesn't bend as easily - probably because I do everything on the ground from the left- so now started lot of carrot stretching and leading from the wrong side etc)
Then once back in the saddle I have cracked down on how firm I am with him in the arena and out and about in our new places!
And guess what? HE GOES! A little squeeze and we are off no need for any reinforcement!
Now I need to get my mum up to date, consistent and as firm so he doesn't fall back into old habits again....
So much progress in so litte time with a mix of everything. I think we've cracked it, so thank you all so much