It could be. I feed Louis royal canin and he weighs 11lbs. I was over feeding him I was giving him 1/2 twice a day. He never gagged he just didn't eat and was basically bloated. He would sometimes throw up though, randomly. I called my vet and he said 1/4 twice a day. I thought that if I followed the directions on the back of the box it would be Ok, but I even minimized it in half which was a cup a day and that was too much.
Here is a website I found on YT that helps me with certain medications. Also try google-ing his symptoms. SEEK PROFESSIONAL advice before trying anything we give you here, Good luck!
http://www.geocities.com/angeljulie2/firstaid.html