Gunk floating on the eyeball or 'crusties' can be caused by a few different things.
Is it a constant build of of crust in the front corner of the eyes, or more like a bit of wet gunk floating across the surface of the eyes? Does he ever get any tears or wetness running down his cheeks?
If it's frequent and quite a lot of buildup, it can be related to teeth problems, allergies, blocked tears and other issues.
If it's just a teeny bit of crust or gunk and it's not happening too often, it sounds like it isn't anything too serious.