Travel advice please

1. My Son has optic pathway gilomas in both eyes and is receiving chemotherapy , although the tumors are benign it’s to preserve his vision. The chemo is for 18months and I am wondering if anyone has been in the same situation as us and managed to book a holiday abroad with adequate travel insurance just incase he fell ill abroad? 
   
   

We thought there is no way we could go abroad but in 4 months time he will have a 3 week break from chemotherapy and if he is well, we would love to book a last minute break in the sun. 

Also does the travel insurance cost a stupid amount of money? 

many thanks