I am very experienced in this area of the law and regardless of what the psychiatrist tells you to prove that you should become the legal guardian for your son (which is what must be done legally in Ontario) you will have to prove that your son is not mentally competent to make decisions about his finances.
If he is making decisions that are not in his best interests but he is still considered competent to make decisions you cannot succeed in Court.
You will need to have him assessed by a competency assessor who is on the list of assessors certified by the government to do assessments for competency hearings in court.
If he is a danger to himself you can seek to have him involuntarily held at a psychiatric unit for an assessment but I assume you know this already.
But the court will not take away someone's rights simply for making irrational decisions about their finances. Just as a gambling addict can gamble away their money so can your son spend it all unless you can have him assessed as being incompetent.
Then the court would declare him to be incompetent and appoint you or someone else close to him to be his legal guardian.
So you will have to prove that your son cannot understand the relevant information or appreciate what may happen as a result of decisions he makes or in fact does not make about his finances. It cannot be that he loses his rights because he spends all his money on drugs.
You can contact the Law Society and use their Lawyer Referral Service. You will be given the name of a lawyer and can consult with the lawyer and the first half hour will be free.
The number is:
1-***-***-**** or(###) ###-####(within the GTA)