Is a solenoid considered a DC Motor?
You are on thin ice.  I agree with your advisor.

What you should do is go back to your advisor and say that now that you have gotten further into the design, experimentation, and testing, that you feel it is more appropriate to use a solenoid instead of a motor.  Depending on what lesson he is trying to teach, he might say that's fine, just revise your documentation accordingly, or no you can't do that since a motor was already specified and it's too late to change that.

If the second answer, suck it up and use a motor.  Then learn from that to not write tight specs next time until you have done proper design, experimentation, and testing to know what exactly to spec.  Writing specs properly is not trivial, and may in fact be what he is trying to have you learn.  This sounds like a good lesson that early on in the process you spec what needs to be accomplished, not how.  The how part happens later as part of the design.