I'm not sure what you mean by “lead balloons” (english isn't my first language so I'm not familiar with many expressions) but I'm very happy with the book suggestion! Thank you! I googled it right after reading your reply and it seems it could be highly beneficial in our situation. Not only for him, but for me too, as I'm currently struggling with my kinky desires and wish to get more enjoyment out of vanilla sex. In my quick overview I read it's focused on women's arousal though, which makes me wonder if it's as relevant as I first believed. But from what you've described in your post I feel I shouldn't worry too much about that. Especially the part on communication seems perfect!
I also love your suggestion on making these lists. Since he doesn't know himself that well sexually, he has trouble with the “Why's” of his likes/dislikes. I think this assignment will be a great tool in getting to know himself better sexually. I will definitely suggest this to him!
He agreed to go to the doctor next week, I'll be making sure it actually happens this time. We actually started working out 3 / 4 days a week recently, he's lifting weights mostly, which I also read can boost testosterone production. I think it's still too early to judge if this is helping or not since we only just started last week. Time will tell.
I have hope the book you suggested will help us identify and understand his context. I have the feeling it's going to be hard to find ways to maximize his desire, let alone getting him to do these things, but we'll take one step at a time I guess. We'll be reading the book first, if we hit a wall I might contact you again if that's okay.
Thank you so much for this incredibly helpful advice! I'm very grateful you decided to read and reply, I feel your suggestions will be a road to succes :). We definitely tackle this together as a team, sex is a team sport after all. I'm glad to hear your wife and you found some understanding this way and were able to work things out together. It really gives me hope we can improve this. Thank you again! I wish I could give you more upvotes!