I'm so sorry you're in this tough situation. You said that you should have noticed that he wasn't happy but again, most often, nobody really can. When I learned about my mom's experience with depression, I felt the same way and even blamed myself, but I want reassure you than none of this was caused or could have been detected by you. No parent wants their child to worry about his kind of stuff.
Quickly, for reference- my TL;DR background: I'm 21F now, but when I was 10, as a result a shitty marriage and subsequent divorce, my mom was flung into a severely depressed state and attempted suicide. Fortunately, she was unsuccessful. I was feeling pretty similar to how you are now. But since then, I've always made the effort to maintain a strong and very open friendship with my mom rather than the traditional parent-daughter. My mom tends to be somewhat pessimistic and still struggles with anxiety and depression. Daily, I encourage discussion about her stressful job, her general stress levels, her diet, mindset etc. My little sister (I'm one of two kids too) causes my mom a lot of stress and I try to give her advice on how to deal. In general, I try to make my mom not worry as much about the terrible shit of the day and make her laugh, find enjoyment in little things, be silly, encourage her to pursue her hobbies by showing my own personal interest in them. (She really into needlepointing so pretending to be interested can be a little tough sometimes...)
I don't know about your exact relationship with your dad but it seems yall are very close. If you can, to the extent you can (because you are most likely going through your own stresses-- I didn't like being 17 very much), make it an exercise to talk to your dad about his day, job, interesting stuff he did, even your brother-- try (if you can) to help him talk it out. Talking out every day stresses can really help with day-to-day anxieties which helps in alleviating some anxiety long term. It might not be deeper conversations (about sadness or even the things he wrote about in his journal) but I think eventually, as you grow and you and your dad's relationship grows into a deeper relationship, it will help him and even help you feel a lot better.
Also, don't feel like you can't complain about your day. Continue to ask for his advice for your own stresses. It will make him feel better knowing that you need him, value his advice and that he can help you feel better too. Encourage him to pursue his art whenever possible! Maybe, to start- ask him about that cat and dog cartoon you were looking for and if you could have it or one like it!