I haven't had a similar experience with that but I would worry about your sons lymph node in his neck as I can feel them in my sons armpit groin stomach and neck went to the doctor and was told as my son is skinny you can feel them under his skin and it's normal enough 

Thanks - he's had it since March and ultrasound has shown it's over 2.4 cm so even though he is super skinny it is larger than they would normally like which is why they want to do the biopsy - bloods all clear other than slightly anaemic and biopsy looked ok other than the size. I'm trying not to google too much because I am worried it could be something more sinister. He gets very sweaty at night - but then so do most toddlers. So hoping it's reactive - except he hasn't been unwell at all - esp with lockdown - we haven't seen anyone!