I'd be upfront. Tell the second interviewer that you have been offered a position that you're seriously considering and that if he is considering taking you on, you require a decision immediately. It's the truth, not a threat.
On the other hand, if the offer you have is all that you want, why even bother with the second one?
As for your current employer, your warm relationship doesn't extend to paying you enough to cover your expenses. While it's nice that you should consider the implications of your impending resignation, your primary responsibility is to yourself.
As the song says, breaking up is hard to do. Bite the bullet and be honest with him. His life will go on (is that another song?).