I’m going to agree with @knitfroggy and @casheroo that you should talk to the pediatrician. If your son is otherwise healthy and growing at his own rate, there probably isn’t a problem.
Does your son drink a lot of milk or juice between meals? Those things could really dampen his appetite. Try limiting fluids other than water, especially in the hour before a meal. Same with snacks. You might also ask the doctor about a multivitamin for your son, if he isn’t already taking them.
As for the Pediasure, I’ve never used it, but I think I would probably offer it to my child after meals. Give your son a chance to eat some variety, and if you feel he isn’t eating much then supplement with the Pediasure. Keep in mind that toddlers have very small tummies and won’t each much at one sitting. Good luck!