Wow, I am pretty surprised about everyone's responses. I am a physician, so feel that I can offer some much needed advice. This moveable "knot" is very common, and very normal at his age. The medical term for it is called "gynecomastia". Boys that are about to start entering puberty, will develop breast-buds for a short-time. Especially if they are already a bit heavy to begin with.
Let me quote some literature:
"Estradiol is produced from testosterone in male puberty as well as female, and male breasts often respond to the rising estradiol levels. This is termed gynecomastia. In most boys, the breast development is minimal, similar to what would be termed a "breast bud" in a girl, but in many boys, breast growth is substantial. It usually occurs after puberty is underway, may increase for a year or two, and usually diminishes by the end of puberty. It is increased by extra adipose tissue if the boy is overweight.
Although this is a normal part of male puberty for perhaps half of boys, breast development is usually as unwelcome as upper lip hair in girls, and can be removed surgically if the boy's distress is substantial."
I would go ahead and make an appt to see his pedi at some point in the next couple of weeks, but try not to stress out until then.
Hope this helps