OP I posted up thread about the situation, but I didn't actually offer you any solutions to make him feel like he matters to you, that he belongs. Off the top of my head:
Push a note under his door that just says 'I still love you', signed whatever he calls you. Or 'I'm going to the shops, is there anything you want me to get?'. Whatever occurs to you at the time, react to the situation as it is at the time, but keep at it.
Get his dad to sit in his room with him in the evening. No need for interaction, just being in the same space, DSS gaming, DH on his phone. When son asks what the heck's going on, the response is 'I just wanted to spend some time with you. Feel free to ignore me, you don't have to say anything'.
Give little gifts occasionally, with a note that says 'I saw this and I thought of you'. This isn't rewarding his bad behaviour, it's reassuring him that you care about him and think about him and value him.
For the same reasons, cook his favourite foods sometimes. Ask him what he'd like you to make for dinner. The answer might well be a grunt for the first several times, but you have to keep going. Not every day obviously, that would be overbearing and in his face, but often enough again for him to know he has a place in the family.
That's straight off my head. I'll try and come up with some more. But the principle of all these is that he doesn't have to do anything except maybe give a short answer to the odd question. You and his dad are feeding his sense of belonging. Let him just absorb it.