Recurrent temperature help?!

My little boy is 17 months old, and since September has suffered with recurrent monthly temperatures that reach above 40 Celsius. We have been hospital each time due to him going blue, being sick and not being able to keep the temperature under control. Each time I’m told it’s an infection ‘but they don’t know where from’ and we’re sent home with antibiotics. Surely temperatures this often should be investigated and I’m trying so hard to get someone to listen but they won’t. Does anyone have any advice on what I could do? Feel like I’m at a loss and my poor baby is suffering from these. Picture below of dates of temperatures for reference.