Not too sure about the "mirror his seriousness level" style advice...
I know for myself, about the only way I can face expressing serious emotions is through the medium of humour...
Edit - In fact, if he's expressing emotions at all through humour, then I'd say if it's done sarcastically, he doesn't mean them. If it's not, then he probably does.
Not too sure about this, either...
if he's innocent and genuine, he'll find it irritating. Plus, I've noticed that when ENFP's try to do the "true words said in jest" thing, it often comes off as confrontational, however "jokey" they try to seem... I think possibly because the difference is kinda between "I
could be bothered about this, but I'm truly not, so I'm joking about it to let you know I could be, and might be if you do it again or worse in future", and "I
am bothered about this, but I'm pretending not to be by joking about it". The first I usually pick up on as fair warning, and I'm cool with it. The second puts my back right up, and gets me in a panic cos I feel sure that this is about to become some tedious head game...
The difference in intention tends to usually be pretty obvious, and if you can't pull it off so it comes over as the first of those, or if you do mean the second but are going for the first,
then it's likely to have your ENTP jotting you down as a possibly neurotic head-gamer, and he'll bolt.