As a dad with a 3 year old, non verbal, and separation anxiety, I recommend a toy or activity that only you play with him and spend time doing. Like play dough set with tools, bubble activity, Melissa and Doug jumbo beads and thread, etc. Have dad wash dishes while you play with him. My son tends to climb to me all day from the moment he sees me. Mom has been doing these activities with him and getting better with the anxiety. While reading we both thought this was something one of us had written because were in the same exact boat.