Dr. Liz's response:
How are you responding to your son’s addition of the letter "k"? Keep in mind that the way you respond to him may cause anxiety and perpetuate the situation. At this point in time, there is little you can do other than avoid making him self-conscious and modeling speech without the letter ‘k’ at the end. Handle the situation by repeating back what he says correctly. When he says, “This is the oneK,” respond by asking, “This is the one?” Don’t draw attention to it and see if it dissipates in a few days.
If this new speech pattern continues, contact a private speech therapist and seek a consultation. Also, tell your pediatrician about your observation so that he or she is aware of it. Please keep me posted!