Another vote for "you can't motivate him to lose weight if he isn't motivated himself." I dont believe that you can do that with anyone b/c they won't do it until they are ready. But you can do a few things that will certainly help.
1. Buy and Cook (I am making the assumption that you do these things in your house - if not, start...!) more healthy meals. Pay attention to drinks, foods, condiments, etc. and slowly get all the crap out of the house. A lot of women that I know use weight watchers recipes. Their husbands/SO's love them and they are healthy options.
2. Ask him to take a walk or bike ride with you on the weekends or after work. You can wind down after a tough day/week together and catch up. If you have kids, take them along and make it a family event.
3. Ask him to work out w/you. Have you asked? you Never know until you ask.
If he says no, keep asking. BF still has never worked out with me, but he likes to hang around and offer form pointers, etc. and I notice that it motivates him to work out himself.
4. Talk with him about buying gym equipment, vids, etc. BF recently bought an everlast punching bag and he gets some work in on that every couple of days when he doesn't hit the gym. If nothing else, it is a good stress reliever
5. Rather than WITHOLD when he doesn't, REWARD him when he does work out. Positive reinforcement!
Christine