I don't think he particularly cares about Bernie because he's a non-threat. He's terrified of losing to Hillary, though.
Losing to a female who is very smart and much more political savvy than he is would be awful for him, given his views on females. Also given the fact that he hates to lose.
^^This.
His willingness to debate Bernie is more or less a sign that he knows he is going to get crushed by Hillary in the debates so he really has nothing to lose by debating Senile Sanders, and Bernie's willingness to debate Trump now is just a sign that he knows he won't get to debate him in the upcoming months
I think Trump is only offering to debate BS because he wants to kiss up to him and try to lure his ignorant supporters (the BernieOrBust/BernieBros/Jfern people) over to vote for him over the evil neoliberal warmonger corporatist corrupt hag, which, let's face it, this is all this facade of a debate is going to be is a knock-down-drag-out Hillary piñata party where both candidates are going to do nothing but throw red meat to their base of Hillary derangement syndrome morons.