Have you confronted him about the instagram account? There is a difference between just looking at someones account out of curiosity to see what they are up to and hacking in to see who they are talking to and what posts they are liking. I would worry about the latter. And even though I myself am guilty of not allowing much time between relationships, I feel that you jumped in too soon. Just because having a child in the mix makes things significantly more complicated. The kid isn't going anywhere which means that they will have to co-parent. Will you be comfortable with him being in contact with her for the rest of your relationship?