I'd say the father should set the example without pressuring the son. In the case of my younger cousin living with us, I set the example. Dude used to be shy about his body but now he's always shirtless at home. I thought he'd be self-conscious in the presence of renovation workers recently but we both stayed shirtless as we interacted with them. The workers didn't say anything about our shirtless lifestyle and why would they. I bet they also like being shirtless in their own homes.
My shirtless lifestyle started the summer before college. I took up tennis in a public club near our house and so after a couple of sessions of seeing my shirt get soaked in sweat, the coach told me to bring an extra shirt like he did. I did, so we'd change our shirts around each after training but one day I just decided to take off the shirt I had arrived in and be shirtless for both the warm-up and hitting drills. I could've put the shirt back on after practice but I thought what's the point. It was like that for the rest of the summer and the coach didn't say anything. Today I don't even bother with the arrival shirt anymore. It's shirtless all the way through. It just feels awesome to feel the sweat dripping all over your naked chest as you get active 👍 It's great that the caretaker and a ballboy who both hit with me are shirtless enthusiasts too. My cousin wants to join and I'm like hell yeah 👍
