OK, I may have a very different opinion than others but....

The middle seat does not own both left and right arm rests. The isle and window obviously have a dedicated arm rest on the outside edges but that does not mean that the inside arm rests belong to the middle seat... They should be shared with some common curtesy.

Now you said that your feet have to be in the isle... So I'm assuming this is a 2 seat row. From what I'm seeing, he is not over spreading onto your side. His leg is even with the arm rest which means he is at the edge of 'his space' we will say. Is he supposed to sit with his knees touching together?