He can do exactly what he said he would do: He can deny you permission to change mental health providers and, if you do so anyway, he can initiate probation violation proceedings. It would be up to the judge to determine if you in fact violated the order of probation.

If your assessment resulted in a recommendation that you use the specific provider to which you object, then your probation officer would have a strong argument that your choice to discontinue would be a failure to "complete mental health and other recommended treatment following assessment by a qualified community-based service provider". The detailed facts matter, so this is something you should discuss in detail with your criminal defense lawyer.