Just do it with one hand as your on the computer or watching TV.
I think you will find that is the norm for many 15 year old boys...many older boys too
Whoever suggested "eating" loads of crap and then burning it all off, should keep his "opinion" to himself.
We don't need to encourage youngsters to eat ****e...mainly because it will make him fat, but wont give him the energy to burn it all off and "turn it into muscle". Getting fat is never a good thing, period.
As has been stated, you seem to be doing plenty. When I was 15, I was pretty dumpy and chunky, but now I am pretty much 6ft and nicely built. Your young, many young guys get fat as they are growing up...it's normal mate.
Colin suggested Pilates...very good, as are Tai Chi etc...it was suggested that it isn't really a "guys thing"....neither is Ballet dancing, but my mate was a Ballet dancer. He got absolutely ripped for it, all through school...but at 18 years old, in pubs etc after doing it for 10 years...he was very well cut, and could kick you in the teeth without spilling his pint...immensely strong...think end scene of Billy Elliot.
The most important muscles are the "Core", which is your Abdominal, Lat's etc. If you get those right, you will have what they call "core strength", not to mention a wicked 6 or 8 pack if your lucky.
You can use a Swiss ball, and work off that. Do your dumbell work, bi and tricep curls etc, while sitting on one, so your abdominal will be working to keep you upright. Big ball about 2 foot across, get it in Argos etc.
Contrary to common misconceptions, the "Abs" is actually one slab of muscle, not several...the 6 pack etc, is actually sinew, that increases definition, when the muscle it crosses, increases in size.
If you want to be fit, then explosive power combined with speed, is far more useful, than simply being big.
Look at Bruce Lee...weighed about as much as an empty crisp packet, but he looked the part and apparently, he could handle himself a bit