This is an important and relevant question. This area is certainly complex and you are to be commended for attempting to approach it in a thoughtful and meaningful way. As a mental health professional, I am frequently consulted on such issues and would like to share a few thoughts.
First, you are correct that this needs to be disclosed. Taking medication is something that the other party would expect to know and as such, you would need to inform the other party at some point in the dating process. You did not state this in your question, but aside from the medication question, his general "down" mood would probably also need to be shared. This is significant information which the other party would expect to know. It could very well be that just general mood information ("sometimes I get down") will be better tolerated than saying that I am taking medication for this, but this doesn't remove the importance of sharing the information.
Second, when disclosing that he takes medication, there are a range of reactions that he may experience from the other party. There are those who will continue to do what was done a generation ago and consider this an automatic reason to terminate. Thankfully, Rabbonim as well as the broader community are increasingly seeing this as a factor to explore rather than reacting with an automatic no. For various reasons, medication is more prevalent these days and while it is wise to follow-up regarding the circumstances, it is often not wise to make this into a knee-jerk "no."
When doing research on this question, the following factors are relevant: a) type of medication, b) functioning while both on and off the medication, c) and overall prognosis. Regarding type of medication, there are many different categories, some for more serious conditions which may come along with significant side effects while others are more routine and safer. It is also important to determine current functioning. There is a huge difference between someone taking medication for mood struggles and is functioning well vs. someone who continues to struggle in day-to-day living. Finally, overall prognosis for this condition and this person should be ascertained. One can and should ask for a release to speak with the mental health professionals involved as well as others who interact with the person (Rabbeim, employers, etc.) to clarify this information to the best of one's ability.
Third, while your wife's logic ( "why start this whole mess with medication which may complicate things") sounds compelling, in the long run your thinking may be wiser. Please realize that people ask and will receive information regarding your son, and his current reputation that he is not functioning well will be a greater hindrance. Reasonable people will tolerate a well-functioning person even if he is taking medication moreso than someone who may not be on medication but not functioning well. Therefore, while taking medication may be considered a blemish, neglecting one's mental health can be considered a far greater blemish.
Originally appeared in Yated Neeman