Goddess minded

Sit your sub down and tell him that you want him to do something. Ideally the purpose of this should not be obvious. It isn’t a service to you or something that benefits anyone, and in fact it appears to do little except make him suffer in some way. Afterwards you can tell him it was a sacrifice to demonstrate his devotion to you. And it will also be interesting to see how eager he is to obey you when he doesn’t know why you’re making the demand. 
Here are some ideas for sacrifices: 
‘I want you to skip one meal’
‘You may not use the internet today.’
‘Walk to the shops instead of taking the car.’
If he asks why, simply say ‘Because I desire it.’ 
In the long term, having him make small sacrifices like these frequently will train him to stop questioning you. Ideally, you should be able to make any demand of your sub and he should obey you without hesitation, regardless of what it is. The purpose of your orders is not for him to think about, and once he internalises this you’ll find him less likely to resist your other orders, too. 
Sacrifices like the above will also make him incredibly grateful to you. Now every time he uses the car, or internet, or eats three meals in a day, he will remember that he can only do so because of your generosity and won’t take them for granted.