Hi there, thank you for posting. I hope you can find some useful information/advice here.
I was in your exact position for a long time. My partner was depressed for a while and had zero interest in sexual activity. Being someone with a high sex drive, this was really, REALLY hard for me to accept. I also felt undesired and unwanted. Eventually my partner did get on antidepressants and and she still was struggling with her sex drive. It crushed me.
The best thing I came up with was to separate myself from the issue. I knew, deep down, she still loved me and found me attractive, and the depression/antidepressants are what was making it hard to express that love, physically speaking. It still hurt, but it helped. I also tried to connect more with her in ways that were physical and with non-sexual touching; cuddling, hair stroking, small kisses, etc. She didn't want to go out much, but we found time to go to our favorite tea shop, and we played games sometimes since she really loves those.
Maybe you could do some of these things with your boyfriend. Depression makes it hard to want to do things, so he might not be up for it, but you could do something as simple as sit with him watching TV or listening to music. It doesn't require much effort and you could spend time together. I would also suggest saying what you said here: letting him know you want to be there for the good AND the bad, and that you're in this. Sometimes depression makes it hard to feel love and compassion from others, because it convinces you you're worthless, but keep being consistent with your actions and words and hopefully he'll come to see it. I would also suggest asking your boyfriend to ask his psychiatrist or doctor for a different antidepressant. If it's having that sort of affect on him he needs a different one.
Good luck and feel free to
PM me if you need anything.