Something weird is going on with my dad [39M] and I'm [17M] worried about him.

My mom died when I was 7 and it's just been me and my dad since then. We have really close relationship and have been doing okay but lately my dad has acted really weird. At first the change in him seemed to be good, he started exercising and care more about the way he looked. I thought maybe he felt ready to start dating again and I've told him many times that it's really okay with me if he starts seeing someone. I even asked him if he was trying to impress someone but he just laughed it off and I thought okay maybe he doesn't want to talk about it.

After this had been going on for a while it started to get weird. He really changed and seemed sad all the time. I thoguht maybe he was just having some dating problems but it just kept getting worse. I finally asked him what was going on and told him that he seemed really sad but he just told me everything was fine and that he was just tired from work. Things are normal sometimes but most of the time he seems sad and depressed.

Him being depressed doesn't really make sense though because he's still exercising and taking care of himself and he does everything that's expected of a dad like make dinner and clean the house and make sure I'm okay but he does seem really down. I went into his bathroom a few weeks ago to "borrow" some cologne and I saw some prescription drugs in his cabinet and I really wanted to research what it was because he's never mentioned being on medication. I didn't, but it made me really worried that he might be sick.

Then a few days after that I was a my friends house and I was supposed to spend the night but he wasn't feeling well so I went home and went bed. It was really late and I assumed my dad was asleep so I just went to bed without saying good night. Before I fell asleep though I heard someone come in the door to our house and I realized it was my dad. It was around 2 am and my dad is never out that late. I probably should have said something so that he knew I was home but I was too tired. I could hear him going to bed but before he got to his room I could hear that he was crying.

The next day I asked him again if he was okay and he said yes, everythign was fine. I asked him if he was sick or something and he looked at me weird and said no. I made him promise to tell me if something was wrong but he kept saying everything was fine and that I didn't need to worry about him.

I don't know if my dad is depressed, or sick or on drugs but something is going on with him that he's not telling me about. He's the only family I have left. My grandparents and mom are dead, I don't have any siblings and I only have one aunt that lives really far away and I don't really know her.

I really want to check what medication he's on or look through his office (I know he has a journal somewhere) but I don't want to invade his privacy. I really don't know what to do.

tl;dr: My dad is acting strange, staying out late, crying when he thinks I'm not around, has prescription drugs even though he's says he's not sick and generally seems depressed. He tells me everything is fine and he's okay but I don't believe him.