Actions speak louder than words. I learned that the hard way. You don't need to go and buy something expensive, in fact that doesn't show them much anyways. Cook him a nice dinner, give him a backrub, write him a love letter or go do something together that you can both enjoy and have a nice talk. There are so many things you can do, I guarentee one of those will really show how much you love him. It has to be genuine and come from the heart.
Little things, a favorite meal, a little note, a message, and sex, if not sex then at least oral. I think that alone would brighten his ten ten fold.
I love to write long notes to my boyfriend about why I love him so much.I makes me so happy to see him smile.I also like to look extra special to impress him, and buy and cook his favorite foods, give him massages and lots of affection and sweet talking.