This is a very difficult one to be honest. People cope with things like this in very different ways. If he's cut himself off from everything he may just need time.
The best thing I can suggest is that you keep suggesting things to do to make him feel better, but don't push him to do it. Show him that you are there if he needs you, and that you are waiting for him when he has finished grieving. This does depend a lot on the person, and without knowing him it's hard for me to give a definitive answer. For men, it is often hard to show your emotions especially when it is related to the death of a close family member. There is a lot of pressure on men to keep their emotions in. This can cause a lot of inner conflict when it comes to expressing your sadness. He may feel he needs to lean on someone but doesn't feel comfortable doing so. If you think he may need to talk, let him know you are there for him but make sure he knows you are not trying to make him talk about it.
If he's not interested in any activity he normally likes to do, this may be because he is associating everything he likes to do with the time before the death of his father, which could cause him pain. Try suggesting you do something different, something he has never tried. Take a day trip somewhere neither of you have ever been before. Try taking him places that you know won't remind him of his father.
It can be a long, difficult process and I really feel for you both. But if you show him you are there you will both come out of this stronger and your bond will be stronger than ever.