According to the dictionary, the word مُمتِع can mean: amusing, agreeable, entertaining, nice, pleasant, pleasurable, readable, rewarding, watchable, enjoyable. So, I don't think 'David is fun' necessarily means that he is funny, but rather that he is a fun person to be with.
Thanks. "Nice", "pleasant" or "entertaining" for example sound much better. I never had heard of "a person is fun". But I'm not a native speaker so that doesn't mean that you can't say it.