Iceberg Lettuce is not going to harm him, but it is made up of mostly water and has NO nutritional value whatsoever. If you want to feed him lettuce try the Romaine. A good site with cavy info about care, food, housing, links etc.. is Seagull's:
http://www.aracnet.com/~seagull/Guineas/

As far as him knocking things over...do you have him in a cage with wire bars on the sides? If so, you can buy a lock crock for his food that hooks onto the cage that he won't be able to move. If you don't have a wire cage, buy a heavy ceramic food dish from a petstore(should only be a few dollars) that way he shouldn't be able to move it as easily. If he's knocking the water bottle off, try a piece of duct tape and tape the bottle to the cage. That worked wonders for my mother's gpig.

If you have any other questions, feel free to ask. I did a crash course in researching guinea pigs when my Mother's gpig ( that she bought a month before at a petstore) gave birth to two babies without us realizing she was pregnant!