My chihuahua has gas.?

Did you recently change his food? If so that could be cause or if you are feeding a less expensive food, or a diet that there is something in his food that is disagreeing with him, then that could be the cause. You should probably talk to your vet about it, just to be on the safe side. You might try just feeding him cooked brown rice and chicken for awhile till you can see the vet. That is a really mild diet and is sometimes used for stomach upset.