Friends with benefits?

I’m seeing this guy from work who agreed with me that neither of us want a relationship. So all we do is see each other and have sex. But we do cuddle, kiss, he’s affectionate towards me. We both just want some company, sex and good talks. So I guess that’s what’s happening and I’m not sure what this is called. But anyway, last night he posted a picture. Not just a picture but a sexual picture from an Instagram post he had seen. It was a guy doing some girl from behind whispering in her ear. It kind of upset me but it shouldn’t so I didn’t say anything because it’s not my place. and quite honestly I don’t know why I even wrote that right now because the point of my story is that it’s his birthday today, I don’t have a gift for him but I can get one if I have to? But do I have to? He’s really just someone I’m seeing from work. Nothing serious. He is a really nice respectful guy towards me and loves listening to whatever is going on in my life. So what should I do? And if so, what should I get him? If not, just a simple happy birthday?