How frustrating for you. I def don't think you are asking too much - I'll bet he is where he needs to be when his teachers ask him!
On the other hand - I guess that kids are fairly egocentric at that age and when he weighs up playing vs meeting you - playing wins. There needs to be a greater incentive (in his mind) to make the "choice" to do as his mum asks. Maybe you could visit a park or get an icecream on the way home some days (surprise him) so he'll think twice about NOT doing as you ask as he won't want to miss a treat.
Just an idea - I'm not a mum (yet) but have taught kindergarten!
(never again)
K