Myocarditis treatable. In most cases, roughly 92-99%, no medication is not required at all and even in mild cases does it require some medication; not for life. It is not as life ending as the father claims. In severe cases; is surgery needed and even in less than 1% of cases require a heart transplant. Whether he likes it or not, his wife was informed of the possibility as every person who receives a vaccination has to sign a waiver. The pharmacist is not required to verbally inform them of the possibility as in most cases they do not know the full history of their patient. In most cases there are instant reactions to people with severe myocarditis after receiving a vaccination; which is why it's survivable even if a severe case happens. This is why those who are vaccinated are suppose to remain at the clinic for up to twenty-minutes to ensure they've not developed any symptoms. My sympathies go to the father; but he has no case.
Perhaps if the father wasn't so anti-vaxx; and allowed the mother to actually take her son to a pediatrician more familiar with the child's medical history and ask if he was susceptible to any of the side effects; this could've been prevented. If he was incapable of informing his wife; that's on him, if his wife was incapable of doing the same research and reaching the same conclusions; that's on her... there are always has consequences to irrational ignorance in the rational world.
Like it or not, the vaccine is still safer than contracting covid.
At the end of the day, the best he may hope for is a settlement where the pharmacy pays for the surgery. Nothing more.