I guess I'm a little late, but I just had to respond because I feel bad for him!
I am probably biased in my opinion of what might be wrong because of what happened to me. I have had severe left leg pain, buckling and muscle spasms, eventually becoming partial paralysis and loss of sensation. All of it was because of a herniated intervertebral disc in my lower back that was damaging my SI nerve. It sounds to me like he may have somehow pinched this nerve. Did he have a big fall recently, or does he walk in a strange way or really is there anything off about his locomotion that could be pinching a nerve? A pinched nerve can cause the same symptoms you're describing because the pressure on the nerve comes and goes depending on positioning (at night and in the morning from sleeping in one position), joint and muscle fatigue (at the end of the day) etc.. etc..
Ask the doctor if this is a possibility and ask if they can conduct a neurosensory exam on him.
EDIT: Also, don't want to scare you, but Hip Perthes is a disease that strikes young boys. Not usually this young of course, usually 6-10year olds, but I suppose it wouldn't hurt to keep it in mind and ask about it. It can be corrected with a specific surgery. My little brother had it a few years ago and he's fine now.