My 21-year-old son who attended college in 2018 was seen by a psychotherapist. Can I claim those costs as medical expenses on my return?

Yes, but you will need to enter them as "Medical Expenses for a Dependant Over 18". They will not be added to any medical expense claim you may have for you, your spouse and dependants under 18, and will need to exceed 3% of "your son's net income".

https://turbotax.community.intuit.ca/replies/3282143