My son doesn’t seem to trust my fiance. How to deal this?

My teenage son, who is now 18 years old, doesn't seem to trust my long-time fiance. He’s known about my relationship ever since and now that I popped the question and she said yes, he’s suddenly disapproves of her. His mom passed away while giving birth to him so I can understand a bit of what he’s feeling but what I don't understand is why the sudden change of heart? I’ve tried talking to him about this but he just wouldn’t budge. Any advice?