I agree with the comment above, changing your own personality is not the way to go, and trying to force yourself to be humorous would soon backfire on you anyway. However, you can turn the tables a bit, without altering your own personality. When the boys make an effort at humour then you can show appreciation of the effort they are putting in, even if you don't find the humour particularly funny. Don't overdo it, laughing out loud when you didn't actually find it funny is a fraud, but why not try to smile or say "you're funny" or "tell me a joke"
Also it is posible to direct the humour towards things that you do find funny i.e. "hey, do you like . I think that's he/she/it's really funny".