My friend, as someone who has fought living on the edge of financial ruin for YEARS, I say take him up on the offer. If you are uncomfortable with the lack of a timeline, then set one. But unless there is a reason for you to feel like this will all go south in a spectacular manner in the next year or so, then I say let your guy provide a helping hand where he can, especially if it's not going to create a hardship for him.
It doesn't sound like this arrangement would in any way inhibit your ability to walk away from the relationship if something went wrong. In fact, it might actually make that EASIER, because each month that goes by you will just be increasingly financially secure in terms of savings, etc.
But...if you are truly way outside of your comfort zone on this, I suspect your guy (who sounds totally fabulous) would understand once you walk him through the things that worry you. I suspect he would understand that it's not really about HIM, or how you feel about him...it's tied up with your feelings about your past.
