A few things that may explain what's going on:
1. A child's eating habits decrease naturally around this age, because growth slows down. Since they're not growing and gaining at as fast a rate, they don't require as much fuel.
2. Your son has figured out that you will feed him again and again throughout the day. A baby doesn't get this, and so approaches every meal as if it might be the last. The urgency isgone.
3. With newfound mobility and interests, eating just isn't as exciting as it used to be, when there wasn't a whole lot else to do.
4. Toddlerhood. Your son has figured out that what he eats is important to you. He's also figured out that he has ultimate control over that department. You can argue, beg, bribe, and nag, but you can't MAKE him eat. And he sure can get a rise out of you when he doesn't do what you want him to. This is one of very very few things in his life where he's got any kind of power, and he will wield it at nearly all times.
Your son will not starve himself to death. I swear it. He will eat enough to give him what he needs. Your job is to give him healthy and nutritious food options, and let him do with them what he will. Sit him for meals and snacks at appropriate times during the day. Give him what you want him to eat. And then back off. Either he will eat, or he won't. If he doesn't, then take it away with absolutely no response. Don't beg, don't yell, don't try the "one more bite" trick. All of that lets him know that he has control over you with his decisions on what to eat. When he's hungry, he'll eat. I promise.