Liz, I just have to say that I have been in that situation of dating someone who still has their dating profile up and it really drove me nuts. It does lead to obsessive thinking--I would check to see if they were still going on, wonder who they were talking to, whether they were going out on dates, etc. I'm impressed that you've been able to resist asking him about it this long! But I think you would be TOTALLY JUSTIFIED at this point in asking him whether he considers your relationship exclusive, and if so why he still has his dating profile up. "I will not get serious with someone who has a dating profile up and active" is a totally reasonable boundary IMHO. And he is definitely asking you to do serious-girlfriend-things. Interacting with his kids is nice, but you're right, it is also a big step! Why not just ask if he can meet your needs on this?
The FB pics of the ex I see a little differently. Those are clearly in the past, and they are of his children's mother. It sounds like they are pretty clearly broken up.