Quoting Mama Milk Machine!:" I understand where you're all coming from about my exes, but his biological father hasn't been in his ... [snip!] ... wants to be with him. He's not going anywhere any time soon, and I waited a few months before introducing the two of them."
I'm glad you're in a healthy relationship now but the past turmoil could still have had an effect on his separation anxiety being worse.
It just takes patience and consistency to build the trust that you are ALWAYS coming back for him. Every time you leave make sure you tell him "ill see you later, i love you" my daughter is 22 months also and she was starting to get anxiety over my husband leaving for work every morning (shes a daddy's girl) but she feels a lot more secure when she hears the same "see you later baby! i love you!" i think its just an extension of the daily routine they thrive on.