Why does my 18 month old hold food in his mouth?

My 18 month old baby starting storing food in his mouth a few months ago. I only put one small peice in his mouth at a time. It has gotten worse recently. While feeding him he will just keep the food in his mouth and not swollow it, at first I thought maybe he had a sore throat but its been going on for some time now. If it goes on for too long and gets close to nap or bedtime I have to stick my hand in his mouth and take it out, which is a hugh fight. Any ideas why he is doing this and how to fix the problem.