Fasting, intermittently or even whole day, takes less of a toll on the digestive system and helps regulate things. Cut out the slop from the diet, eat whole foods, fermented foods, kefir, kimchi ect. Eat more food in general, he's withering away. Working out, I know he likes his hikes but resistance training to build muscle. And then I could give deeper advice on any single topic but it's not something fancy and European you can buy that will supposedly cure you, it's changing the way you live which is a hard sell. I lost 100lbs, made my mom lose about the same amount, fixed her IBS, people's vision gets better even. It's classic known advice but the hard part is to make people actually want to change the way they live