I am so sorry to hear your story and see your pictures. It sounds very much like the story of my daughter's leg. Her's is located on the inside of her upper right thigh.
She was about 1 year and a half when her leg got big, swollen and red like that. That is when she was diagnosed with a mixed venous & lymphatic (looks like a rash) malformation with thrombosis (blood clots). She was given a course of antibiotics and the swelling went away. She had an infection and that was what caused the swelling and redness.
She too was not yet 2 years old when they did the MRI under GA. I was a total basket case with worry. In the end it all worked out and she was just fine. But I will tell you that I was a complete and utter mess that day. I know what you are feeling.
She is now 7 years old and coping with her condition with finesse and ease. Her teachers and school nurse are amazed at how non-chalant she is whenever she goes to the nurse to get a dose of Tylenol and Motrin products. We are parents must give them the skills to cope with their condition. (Only after we pull ourselves together!!!)
If it is a Lymphatic Mal - Dr Francine Blei - my daughter's current doctor wrote a parent friendly book to help us lay people understand. There are 2 wonderful clinics in NY that may be able to consult with your doctor's in your area. One is at Columbia Presbyterian Hospital - Vascular Anomaly Clinic and then the office of Dr Blei.
You & your son are in our thoughts and prayers.