Any chance that it's Osgood Schlatter syndrome? Basically, there is a disruption of the tibial growth plate causing the tibia to protrude below the knee after a period of rapid growth. It's relatively common in older children, especially active children. In fact, my husband had it when he was younger. Your son seems a little young, but Osgood Schlatter was my first thought when I read your post.
I second the pp...I had Osgood Schlatters when I was young, about 9 or so. So 7 doesn't seem too young. Might be a good idea to get the Orthopedic Surgeon's opinion before too long. When you press on the lump does it move, or does it seem stuck in place?
I had the same thing as a child on both knees (below) and still do, just less painful and less pronounced. I'm 20yo. I'm curious to hear about the results. Good luck!
Me  DP  out & proud!
Mama to DD1 2007 DD2 2009 DD3 2012
TTC #4 as the "other" mother
3/2015: Home ICI - BFN
On a break - Trying again: late April 2015