You've probably come across such a person who tries to be funny but he can't be because he's not but he thinks he is. Their jokes are old or simply what they say is not funny, you get the idea. Not that they're weird or bad; actually they turn out to be real sweethearts sometimes but they're just not funny. What do you call such a person?
Your grandpa is nice but let's face it: he's [the word]
Oh man I don't know why he tries so hard to be funny! Why nobody tells him that he's just [the word]. That's pathetic!
An adjective, a noun, or a noun phrase will do.
Edit: In my native language we have an adjective to describe such a person which literally means insipid that is used for food, too. Like insipid food which tastes bland, the person who's *insipid has not much sense of humor as if being funny was like spice making the person *tasteful. I thought there might be an equivalent in English but I gave up searching after a while and I posted the question.