I see a clear distinction between the role of grandparent and the role of care-provider.
When I looked for who was going to care for my daughter when I went to work, I looked for a facility that was similar to me.... that had the same ideas about play vs education / snack choices / interaction style. Just like you can't go in and tell a daycare how to run their business, unless you have hired someone to be your employee you can't tell them how to do their job.
You haven't really hired your son's grandparents to be your employees - you are simply paying them some cash to offset their expenses and compensate them a bit for their time. That puts them not in the role of caregiver and not in the role of grandparent.
I agree that I would speak with your husband and determine a date that you will put your son in a "school". But then make sure that you establish consistent times that they can count on still getting to be grandparents so they don't feel like you have shut them out when you no longer need them.
Until then I would not say anything to your in-laws. First of all that's your husband's job to talk to his parents about his son. You handle your parents. But I would tread very carefully because as sure you are that your views are correct, your in-laws are sure their views are correct. After all they raised your husband.
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ETA - I see some posts about when "discipline" starts. Discipline and consequences / punishment are two different things. Discipline starts almost immediately with how you respond to your child. How you handle it when they start trying to roll over during diaper change, or spit food out of their mouth. Discipline is coaching or teaching what behavior you WANT to see.
Punishment or consequences will happen if discipline is not utilized correctly.
For probably the first 4-5 months there really aren't consequences or punishments. You don't put a 4 month old in time-out, for example. However, once they get mobile and more aware you will absolutely start to guide their behaviors.
How do they interact with your son as he wiggles during diaper changes? How do they interact with him when he is "done" with his bottle and pushes it out of his mouth after 2 ounces? what about when he "squeals" or makes very (very very very very) load noises? That is when *true* discipline starts. By gently and supportively guiding your son's behavior and interacting WITH him to engage int he world - as opposed to "reacting" when he does something they "don't like".