Baby BJorn potty seat. They're expensive new, but you can sanitize a hand-me-down one. I got one for 50 cents and it was great! The guard is built in and therefore your son can't take it off, it is a short guard but tall enough to keep from spilling over.
But you can also teach him to sit backwards on the potty- helps him not fall in when he's younger/smaller. The drawback to that is he has to be actually naked from the waist down, rather than pants around the ankles.
My son really just likes "hooking up" to the big potty. That is, he just gets on it like a big person and kind of hangs down in- you know how kids hang on to the seat and droop their butts down in. That is by far the most effective way to keep from making messes.