J.B.
My mother can be like this. It's like holidays and special events trigger her crazy button. She would get all worked up and then unload a stream of awful comments on my siblings and/or me on, say, graduation day. My mom thinks that everyone except her needs therapy and medication so there is no resolution in our family, we just try to relieve the pressure on her and expect that every so often, holidays will include a meltdown and we don't take it personally.
You're probably right in that these kinds of days trigger some sadness that he doesn't even recognize because it's so buried. I would write him a gentle letter letting him know what you think. Include specifics but don't be accusatory, just put together clear picture of the examples. Then invite him to let him know what you want to do. You want to celebrate his milestones and show that you love him, but will respectfully not do that if it causes him pain. Then maybe you can replace a birthday celebration with some other random celebration. Instead of celebrating his birthday, you can celebrate the first day of summer, or open day of a sporting season or something else.