Does anyone know if a higher fat content dog food may be too hard for a dog to digest?

Try a chicken and rice or lamb and rice formula. Or even better, boil some skinless chicken in water without any seasoning or salt. Use the broth to make instant rice. Shred the chicken and mix with the rice, feeding very small portions. You can also check the vet websites for dosage and give him some Pepto Bismol. Stop bouncing from food to food until his stomach problems resolve. If you do not see improvement in 24-48 hours, see the vet.