I know someone once said to use Magnalax or MVT powder mixed in 2 gallons of warm water to help with possible impactions- it is Milk of Magnesia sold for vetrinary purposes. May need to tube him if possible. Maybe ask the vet about it?
http://www.drugs.com/vet/magnalax-powder.html
Here is some info from merck, guess it depends on where the impaction is, what stomach:
"Treatment:
The challenge is to recognize the cases that will respond to treatment and those that will not, ie, to determine those that should be slaughtered immediately for salvage. Cows that are weak, have a severely impacted abomasum, and have a marked tachycardia (100-120 bpm) are poor treatment risks. In cows that are treated, the metabolic alkalosis, hypochloremia, hypokalemia, and dehydration should be corrected. Lubricants and cathartics can be used in an attempt to move the impacted material, or the abomasum should be emptied surgically. Balanced electrolyte solutions are infused IV continuously for up to 72 hr at a daily rate of 80-120 mL/kg. Some cows respond well to this therapy and begin ruminating and passing feces in 48 hr.
Mineral oil can be administered at 4L/day for 3 days. Alternatively, dioctyl sodium sulfosuccinate (DSS) can be given by stomach tube at 120-180 mL of a 25% solution for a 1,000-lb (450-kg) animal mixed with ~20 L of warm water and repeated daily for 3-5 days. This dose rate will kill rumen protozoa. Mineral oil and DSS should not be administered simultaneously because DSS may potentiate the absorption of mineral oil. A beneficial response cannot be expected in <24 hr; in cattle that respond, improvement is usually seen by the end of day 3 after treatment begins. "
Jenna