C.A.
Hi A.,
It sounds like it's time to learn to sink the cheerios. Throw 2 or 3 cheerios in the toilet and teach him how to aim "Mr. Winkee" at them to make them sink. It is a fun way to teach them to winkle in the toilet and to aim inside the bowl.
Have fun most of all and yes it may be a few messes as he learns to aim but overall I have found this very effective in teaching not only my son but most of my nephews how to sink the cheerios.
C.