HI M.,
Please get the book "Child of Mine" by Ellyn Satter. It might be at your library, and I know Amazon carries it. It is the most wonderful reference I've seen on feeding kids up through kindergarten and beyond. You'll feel much less stressed and more confident after you read this, believe me! Kids your son's age are starting to really get engaged in the world around them, and so get distracted by all the lovely new things they are noticing.
One thing that really struck me in your post is how much effort you are putting into getting food into your son. Please turn off the TV and have mealtimes be a calm, social event. If you're "sneaking" food into him while he's entranced by the TV, he'll have a hard time down the road learning to regulate his food intake himself, because right now, he's not eating according to his own hunger. He might not eat a whole lot for the first couple of meals (or days!) on your new routine, but believe me, he'll eat enough for his body after a day or two!
I worried about my son not eating enough at this age too, especially since he's always been super slender (90th % for height, 50% for weight), and I felt much better after reading this book. My kid is now 4, eats great, and is still very slender. It's just his body type.
Best of luck to you!
C.