Lifestyle Changes

Does anyone have any tips to change our lifestyle together? We’ve been married 28 years and we’ve become quite complacent with our diet & exercise.
I have a really active job but my husband has an office type job with lots of driving & desk based Work. I’m the one who does 99% of the cooking & organising meals, I tend to cook to his tastes that I know which are quite different to mine. In our own ways we can be quite fussy. Over the years it has caused many an argument mainly from me as I’ve been desperate to lose weight. On several occasions I’ve joined slimming groups and he’s supported what I do but it never felt right! I’d lose then put it all back on!
Recently he said he needed to lose weight (a first!) and I want to capture the moment the best I can! I asked what he planned and what we could do together. He wanted to join a slimming group and I think I’ve over done them so much I don’t want any more money going into their hands! For Christmas I bought him s cookbook based on healthy eating, he’s highlighted recipes he’s willing to try which is a step in the right direction but I don’t want to lose momentum. How can I motivate him & myself to do this, we both need to lose over 40lb. Any help or advice would be very much appreciated.