I assume your garage has electricity... If both of you die from this disease it's not going to help anyone. After 19 days he can move into the house. You can cook for him if you want to but food should be left outside the garage door and be in a throw away container... This virus can live on plastic and stainless steel for days... He can buy a bed at a thrift store... and donate it back when the two weeks are up...
If he's a family member- and I assume he is - and he does gets sick - - you'll need to care for him. Make sure HE wears a N95 mask to keep YOU from being infected.
It's better the garage is a little cold than too hot like in Florida - you can buy a $30 space heater on Amazon but not a $30 air conditioner... Of course we'd put a person in a screened in outdoor porch...
Remember, you're no help to him if you're sick too... and it's only a short few weeks he'll have to live in the garage. Stay safe ...both of you.
You said two weeks and you said 19 days. Do you think he would need to be there as long as 19 days. Thank you for your time and concern.