Oh. Ok, well, I guess someone forgot to tell my son and most of his teammates this

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He played baseball starting with t-ball at 4 and started flag football at 6. He continued to play both sports (plus basketball for awhile) until high school and baseball through high school. He quit football because he was injured and his baseball coach wanted him to. He practiced the way the OP described at 6 years old and more-much more. He still loves sports and would be playing right now if at all possible. The boys he grew up playing on the team with are almost all the same way. You would have to know my son and these other young men, but there was absolutely no parental pressure involved. In fact there were times I would have loved for him to choose to quit, but he just loved playing too much; it was and is too much a part of him.
OP, if my child was asking to not go back; I wouldn't push him. It really should be up to him on that. And if he does go back, just watch him and you will be able tell if it is too much for him. If he just seems overly tired or stressed then it very well could be too much.
These "practicing" may not be actually running up and down the field and running plays. At 6 years old, they have a lot to learn about football. The coach has to start with the very, very basic skills of the game. This practice may not consist of as much physical activity as everyone is assuming it does.