Applying to summer research project, should I tell prof about mental health issues to explain poor grades last year?
If the lower grades belong to a specific time period, then it might be worthwhile to provide an explanation.  If they are scattered here and there through your first four semesters of college, then maybe not.  Keep in mind that many students have a bumpy start to college, and professors are used to that.

At this stage, perhaps you could leave it rather vague ("health problems") for now.  If you are accepted for the summer project, do mention that recently you were diagnosed and began treatment, so that your professor is aware of the general situation.  Also, if there are any specific accommodations you would find helpful during the summer project, let him know what those are.

Speaking of accommodations, if you are in the U.S., but don't have ADA accommodations (also known as 504 accommodations) set up, this would be something to consider looking into.  If you are interested, you could start by


looking at your college's office for students with disabilities web pages to see what documentation they require;
thinking about what accommodations would be most helpful for you.  Look around on the web at lists of accommodations some people have found helpful, to get some ideas.