This is a very common thing that we see in families, particularly if people are not talking openly about what's happening to them. Parents want to protect their children from being upset and children want to protect their parents from being upset and so we often find that everybody try to keep up a brave face and not talk about what they feel and things they are worrying about. So when we don'talk about what's really on our mind and what's really important for us, then we often find it very uncomfortable and difficult to talk to each other at all and the atmosphere can often get very strained and tense.
Try and communicate openly about how you feel and what you're thinking about. Be honest and don't worry about upsetting your dad. He will already be upset too and he'll be worrying about you and if you don't say anything but pretend that you're doing okay, then he'll try not to bring anything up in case you get upset by that. Then you see, you can get into a circle where you both think and worry about the same things but you both keep it inside you to protect each other.
Things that are bottled up only gets bigger and more difficult to deal with and although it can sound a bit simple, talking about things will ease the pressure as you are venting your emotions instead of them just building up inside you. Just tell him that you don't know what to say but that you'd like to talk about what's happening to him and how it is affecting you all. If you share your worries you can support each other instead of carrying a burden all alone, a burden that's just getting heavier and heavier.
Once you get over the initial difficulty about starting to talk it will get easier and the imaginary wall between you will disappear. You'll then soon be able to talk about what's bothering you as well as just normal things that go on in your life.
Good luck with opening up!