Sounds like you've all done everything you could, sometimes the best thing is to stop suggesting remedies. Don't we all get tired of advises and suggestions sometimes, no matter how good they are, sometimes we still feel like nobody understands, especially a teen mind. Sometimes a push to imporove can have a negative response. That I can tell from my own exerience as a teenager. I used to close myself in a room even when I had my family all in one house. I used to hate anyone inviting me to join them in the living room and watch a movie together. I was stubborn for no reason, and grief can only make it more intense, but it may not be as uncomfortable for him as you may think.
I would also suggest thinking of what characteristics of his father he is missing the most. Maybe there are other men that can help slowly present father like role in his surroundings. Like for example my friend's father looked after himself really well, he was extremely fit, always well groomed. Now my friend finds comfort in exercising, she enjoyed personal training a lot, which I think is inspired by her father in a way.
Perhaps you can let him go on vacation by himself, not sure what options there would be in this regard... Surfing camp for teens, perhaps hiking trip... Whatever he would find appealing and perhaps he does need to do it alone, as he clearly doesn't connect with anyone at the moment. He will need to take his first steps himself.
Wishing you all a lot of patience, and strength
