I'm not going to do anything with him as he doesn't really want to, but it doesn't feel right to just ignore him all day and let him do his own thing. How does one check in without seeming like I'm taking pity on him? Birthdays are always huge for me so this is new territory, and he is an extremely close friend who I talk to everyday. What can I do without making a big deal of it but also ensuring he knows he is appreciated?
EDIT: I did not say I was going to celebrate with him. I said I just don't want him to feel lonely. I was expecting suggestions like "just give him a 2 minute phone call or a quick message" (for example, but was hoping for other options too) not "you're a terrible friend for not wanting your friend to feel left out". I'm hardly throwing a party or shoving $456894 worth of gifts down his throat. Damn!