dude, that is a touching story...
DNA Test is a MUST if you wish to take this down the legal route, your ex obvs slept with anyone that winked her way, so you have no way to prove this little lad is indeed your own, my aunt adopted a little lass and she also was a spit of my uncle, my cousin had a lad that has birght orange hair, her and her husband are as blonde as they come, POINT > looks mean nothing in new born.
Dont try and get even with her new fella, let him deal with it too, you owe him nothing and she must-ant care about others feelings so it wont bother her even if this new chap upped and left.
Make a note of every communication you have with her and seek legal advice.