Make up a 'coupon book' that includes things that you know he will enjoy or appreciate you doing for him that he can cash in at any time. It has worked well for me and my partner in the past :)
Just be there when he needs you
Take him on a nice vacation somewhere new.
Listen to him when he speaks. If he has a show or sport he likes, join him in watching it. If you do not know it, watch him also and when he responds to what he is seeing, ask him about why that was so good (or bad). Make any questions specific so he will know you are really interested in what he is. Whenever you catch his eye smile and nod slightly just acknowledging he is there and you care. A smile and perhaps a kind word can go a long way. Be open but not pushy and then accept what he shares with you. When he does share something with you, it is because he wants you to know. He may not make a big thing of it. But remember what he shares. He has a lot he cannot easily share and the more you accept, the more comfortable he will be. This is of course assuming you are doing the normal things and he enjoys being with you. These are just somethings to make a good thing better for him. And be sure to appreciate all he does for you. Nothing is more fustrating than to do something for my lady and she is not even aware of it or ignores it.
Nothing like keeping a perpetual smile on his face. Us guys really are easy to keep happy.