Perseveration is what happens when a child is "stuck" with a particular idea or action. The best treatment is via a combination of judicious medication (such as serotoninergic antidepressants, antipsychotics) and psychotherapy. Some tips that may be useful for you:
i) Talking about it: talk to the child when he is not perseverating, describe the observed behaviour and situations in which it causes problems.
ii) Empathic understanding - this will reduce anxiety and calming down may help in getting the child unstuck
iii) Self-monitoring to learn and identify potential triggers and also what helps him to stop
iv) Create a "stuck signal" - a simple phrase or action to let him know when he is stuck.
v) Create a plan to get "unstuck" - identify things he can do when the "stuck signal" is given. Eg. a time-out, doing another activity etc.