Sounds like you already have alot of stress in your house. No matter how much a family has it together it is hard to have grown kids and their children living in your home. Men can get just as insecure as women can, and when we are insecure we look to our spouse for support....be thankful he was able to share his feeling with you instead of hiding them inside and letting the brew.
Sounds like the two of you need some time together, take him out on a date. Let him know you WANT to be on HIS arm, no one elses. Remind him that you are losing weight for your health, which will allow you to be around longer for HIM and your family. Ask him if he would like to join you, tell him you will support him and help him every step of the way. Maybe he does not want to go on the Alli pill, but would be willing to go on the meal plans to match yours. You can be a team, he will certainly benefit, and so would your relationship.
Take him to a movie, or maybe to a late evening picnic in the park, pack a special dinner that fits into your plan. Leave notes for him to find during the day.
Remember times when you have felt insecure and look for opportunities to build him up, just as you would want him to do to you.
They are a different creature, and yet in some ways very much like us.