My mother is the same about the next stage up seats, I have an 11 year old sister and she is adamant that she is just going to have the seat she had for her in her car (she will be looking after the baby 2-3 days a week when I go back to work). She refuses to listen about the expiry dates because she knows she hasn't had a crash, so when the time comes to move him up into the next group seat I will have to find the money to buy her a separate seat for her car I think.
I agree with what everyone else has said, find some statistics and facts to show your OH (that's what I'm going to try with my mum) and hopefully that will make it click. If needs be I am sure there are some horror story articles about accidents in old car seats, a tough love shock approach might help.