Antidepressants and sex

So my boyfriend is on antidepressants and says he doesn't feel like having sex with me and he's not sure if it's because of the antidepressants or because he's bored with the sex. I'm able to get him hard and that's all fine and he said he's still attracted to me but he just doesn't want to have sex. If he's bored with the sex, how can we do something to make it more interesting? He's still has injuries that can give him a bit of pain so we are restricted to certain positions, we have tried different places but he is also self conscious so again that limits that. I've tried sexy outfits but honestly can't afford to continually buy them. He's suggested a threesome but I'm dead against it because our relationship is already fragile. And we tried anal a few times but I hated it and get hurt doing it. What can I do?