R.K.
My ex is like this. When we first split up for the first 2 years he didn't date anyone and cared less whether or not he saw his son then he dated this girl and he all of a sudden wanted my oldest all the time. Well it turned out that she had a son close to his age so they occupied each other so they didn't have to sit down and play w/ them. He split up briefly w/ her this summer and started ignoring his son again and then started dating her again this fall and started wanting to see him again.
My only piece of advice is that if he wants to be a dad let him but do not let him yoyo your child back and forth being there/not being there/ being there..... that's worse then not being there at all. I keep conversations w/ his dad as short as possible so that they remain civil. I usually put a note w/ all my concerns I had that week about him/ any meds he is on, homework, sports, etc in it for him so that I don't have to sit there and tell him every little thing bc I quite frankly would rather not speak w/ the @$$.