OK, I would not ever ever force feed any reptile pure puppy food. There are recipes for feeding reptiles that include puppy food but not pure puppy food. Here is one that is commonly used is leopard geckos:
http://www.faunaclassifieds.com/foru...5&goto=newpost
Now, when I imported Jacksons and treated them for parasites I used Jump Start if they had problems eating after being treated for parasites and I did not "force feed" them, I placed a small amount on the top lip right at the opening of the mouth and they would want it off the lip and open the mouth, then I would squirt a small amout (maybe dime sized) into the mouth and I never lost any of them and imported close to 200 a month for several months and kept them for 60-90 days before selling any.
Force feeding will more often than not take up more enegry from the reptile fighting it than they get from what you forced them to eat and puppy food is not a good source of nutrition for a chameleon.
Id honestly try some different food items (silk worms, wax worms, grapes, greens, squash, roaches etc), natural sunlight, more mistings and better calcium if he has a calcium problem.
I mean think about it, do you like McDonalds Cheeseburgers enough to eat one every day for every meal you have, chams are no different, they do not want crickets and mealies for every meal every day. They enjoy lots of different food items and get bored with a lack of variety. That is one of the great things about veileds, they love a lot of different food items.
Im not sure how a heating pad will raise the temp in a screen cage much but you can try it. When I have to raise the cage temp inside the house I use a light fixture with a blue or red bulb at the bottom of the cage since heat rises or you can raise the temp in the room by upping the heat in the house.