Remove contact from iMessage "memory"

I have two iPhones, one provided to me by my employer and one I use personally. My employer has both phone numbers for me in his contacts. Before I was issued the company phone, he contacted me via my personal phone number. I have asked that now that I have a company phone that all work-related messages be sent there, however I am still getting messages from him on my personal phone when he clearly meant to text my work phone. He claims that he has removed my personal number from my contact but messages are still going to my other number. I believe this is because he still has old conversations in iMessage that contain my old number. Is there a way for him to reset this so he only has the correct number saved? Is there a way I can block him from texting my personal number, but so that he knows the message did not go through?