My DH can be similar if he's doing something he's involved in. He can ignore the kids unlike I can. I just can't ignore them calling me and I hear everything they say and every sound they utter, all the time, no matter if I'm involved in doing something else or not. I think it's just the way men are programmed. They aren't able to do two things at a time, so if your HD is involved with his computer or whatever, he is just not able to be listening to your DS too.
Have you tried just asking him outright to sit with you both for 15mins before he goes off to do his own thing. I know my DH like time to be able to unwind after work. He will help me bath and dress the kids but then he wants time to be by himself for a while. Which I can understand. He has just worked 12+ hours and wants to have a few mins to think!! If you havne't brought it up with him he may not realise he's doing it and might think he's paying attention and playing?