Consider having him help you "cook." At five years old he is just about the right age to follow simple directions with your guidance. Get a kids cookbook and flip through the pages with him. Let him choose a meal with the agreement that he will at least try a small portion. Teach him how to make a sandwich, maybe open face with peanut butter and bananas, or a tortilla wrap wtih peanut butter and jelly. Pizza is a another great joint cooking project, and a fun way to sneak in healthier food, just remember to finely chop things,and you might have him make faces out of the ingredients. Just don't expect a mess-free time in the kitchen!
Also, remember this: at this age kids are beginning to assert their independence from Mom-you can lead him to the table but you can't MAKE him eat, to paraphrase a famous proverb. He is testing his decision making and preference choosing skills. But you can set limits-like letting him know that "this" is what's for dinner and he is welcome to join you or wait til the next meal. Stay calm if he balks. Empathize when he decides to leave the table only to come back hungry later, but let him know that dinner/lunch/breakfast is done and he will have to wait until the next meal. Give him choices, but limit those to two only, ie; "Okay, you can have a grilled cheese sandwich with carrot "fries"(cut the carrots to look like French fries) or you can have PB&J with ramen (a smaller helping than if he were to eat the ramen alone)." Try giving him smaller portions of the new or chanllenged food you want him to eat, but then let him take it from there. If he fights it, don't worry. Kids will not starve themselves. The Pediasure thing is a good suggestion, but you might also try fruit smoothies (you could add some carrot juice or freshly juiced carrots for some added nutrition).
My best advice is to avoid making him overly self conscious of his eating habits. Make meals and eating a seemless and natural part of your daily routine, and make sure he sees you enjoying your food without being obvious about it. He may always be rather picky when it comes to food, but when you think about it aren't we all?