How can I motivate my husband to lose weight?

Enlist help. Find a doctor who can help you by talking to him very seriously about his weight. You may also consider a dietitian, personal trainer, or even a trusted friend to talk to him also. Sometime it is difficult for a spouse to be the motivator. It may seem like nagging instead of caring/love. The most important thing is to not give up. If one tack doesn't work, take another. Good luck with it.
With enthusiasm. Point out that obesity, and the diseases for which obesity raises his risk - hypertension, heart disease, obesity, stroke, etc. - are very likely to kill him early, taking him from you and (if you have them) your children long before he would otherwise pass away. The number one thing he can do to improve health is lose weight by watching quality and quantity of food intake and increasing exercise.