Need a little advice for excon relative.

Context and explanation:

As some of you know, I took in (won custody/adopted/whatever) a relative that was released after a 35 year stretch.

Initially, he refused help and stayed on the streets for a couple of years, during which he got a traumatic brain injury. Started showing signs of Parkinson (now diagnosed), and cancer (in remission).

In-laws finally got him to stay with him, but, let's just say it was a stressful situation overall due to their military and law enforcement backgrounds.

We convinced him to stay with us, and aside from walks and playing with dogs, he's quite isolated socially.

He refuses to join us, and cooks (disconnected oven for safety) for himself in his kitchen.

He's always been a loner even before prison, but he's having issues adjusting to life and need to know how to bring him out a tad without it turning into an argument.

I know there are a couple of groups that might help, but simply refuses.

Any suggestions?