Hello,
6th house is studied for health issues. In his chart, Ketu and Shani are located there. Here are readings -
https://www.indastro.com/house/saturn-6th-house.html
and
https://www.indastro.com/house/ketu-6th-house.html
Current dasa is for Shani (2010 Jun - 2029 Jun). This may bring health problems as shani sits in 6H.
6HL Mars is in 9H which means his health condition may impact his luck, foreign travel and his father.
For transit shani in Gemini from 26 Oct 2017 - 24 Jan 2020 :
"If Saturn transits to ninth house from natal Moon, you might begin to feel distressed and worried all the time. You may change your job frequently during this period. Chances of long distance travels could form too during this period. To maintain peace of mind, you should practice the path of spirituality consciously. Your expenditure could also shoot up so keep a watch. You might feel the job market to be very competitive too. Some issues with children and spouse may also persist."
Hope he gets well soon.
TC