Was I wrong for teaching my son "poor work ethic"

Hi! I (38m) have a son (16m) and my wife (38f) is pregnant with my second son. My son lost his mother 5 years ago in a really bad car accident. Both his mom and step sister passed away and he broke his back. I was a single dad and had to learn how to be both a disciplinary figure and a nurturing figure. Sadly I didn't have most of a role model growing up on being nurturing so I just did everything my dad didn't. I may be a little too lenient at times but i try. My son works at Walmart. He is a good worker. Never calls in. Is always on time and works hard. Ive always told my son to make sure he is taking care of himself over a job because if you were to die tomorrow, your job would have your position up on indeed before your obituary was out. Sad but true. Because of the accident he has chronic pain. Mainly in his back. This morning I went to wake him up from school and he had been laying on his stomach on top of my wife's yoga ball all night because his back hurt so bad. I kept him home from school. He asked was it okay to call into work. I said yes. Well. My dad came by to help me with my wife's car and i informed him on my son's back and its hurting again. However when i told him I let him miss work my dad got so mad and said this generation already doesn't want to work and the fact that I'm letting my son miss work is teaching him bad work ethic young. He asked will he call in every time his back hurts. I told my dad it's chronic pain. He's in pain a majority of the times and just lives through it. It's one of his bad days when he can't. Well my dad lectures and my son about how he left his boss short handed and how calling out for any reason other than an emergency is unacceptable. I told my dad he can have his opinion. My dad said it's not an opinion its a fact and told me just because my son's mom died doesn't mean I need act like a coddling mother and I'm setting him up for failure as a bad husband and bad provider because of his work ethic. He's been with Walmart for almost a year and mever called out once. Is it really that big of a deal