It could be many tihngs. if he is purring he may have problems swallowing at the same time.
He could have some hair in her throat, or some grass or a bit of food (bone) stuck in there. It could be worms or it could be something more than that like a throat infection ...... and so it goes on.
Just keep an eye on him (as I'm sure you do anyway) and if he seems off colour get him to the vets. Has he be wormed recently?