we have started trying to address this with dd1.
We do lots of different things to try to get her to imitate. Motor imitation is her worst problem (after speech delay). She is, surprisingly, very good at verbal imitation, and so we mix a lot of that in to keep her happy.
we have:
used hand over hand for immediate results - touch head/shoulders/feet/tummy etc. lots of praise (dd1's biggest reinforcer) even when we have helped (made) her do it. then try ahgain, and lots of praise if she even twitches in the right direction. repeat endlessly.
I have also copied wht dd1 was doing (eg tapping on a table). Waited until she acknowledged I had joined in, then stopped. She stopped too a surprising amount of times. Then I would start up, and she started up again, etc. Lots of reinforcement again.
And, have you tried with something your ds is interested in? Dd1 loves stacking, and so at times, I would sit with the stacker and stack up shapes. Dd1 sometimes came across to join in (I had not asked her over/told her to come and play etc - she was purely watching and then copying my actions)
Technically, I think, when working on imitation, you should (if verbally prompting) accompany with "do this". this does not work for dd1 at this point. She works much better if we name the action - she will then imitate after a few attempts, and then we can fade the verbal prompt completely.
Imitation does not have to be big - would your ds copy you rubbing on a chair arm, for eg, or banging a drum?