Well, I really don't know how you "force" a baby to eat, short of taking them to the doctor to have a tube put in his stomach. I know it is important to feed baby healthy foods, but have you tried something a little more treat-like? Give the kid a cookie and see if that perks up his appetite (and thirst.) Applesauce? I wouldn't introduce a whole barrage of new foods all at once. I don't know what you've already introduced, so I am kinda shooting in the dark. Gerber makes fruit and vegetable yogurt melts that my kids ate like candy. Do you offer your baby a sippy cup? At this age, he may prefer it to bottle. I transitioned my daughters using the Nuby Sports Bottles. They loved those.
At 13-months old, some babies are too excited about their newfound mobility and other skills that they don't want to sit down and eat so much. They're too busy exploring the world. It is not unheard of for them to loose a little weight, but if you are worried your baby is loosing too much and not eating enough to sustain himself, take him to the pediatrician. He may have a stomach bug going on. Also, it is allergy season. If he has post-nasal drip, that can greatly effect the appetite.
Good luck to you. It is no fun when we feel we can't feed our kids.