You don't need a GAC to use either a wheelchair or an
ECV.
That is true.
Guests do not need a GAC (Guest Assistance Card) to use either a wheelchair or ECV in line. GACs are a communication too for things that are invisible and the Cast MeMbers would not know about otherwise. Most lines are wheelchair accessible to the point of boarding; if there is a different boarding area or path for wheelchairs or
ECVs, the CM can see the wheelchair or ECV to direct the guest where to go - no GAC required.
There is more information about GACs in post 6 of the disABILITIES FAQs thread. You can find that thread near the top of this board or follow the link in my signature.
To answer your question about when Guest Relations opens, it is open whatever hours the park is open for guests to be inside. There is also a Guest Relations window outside of MK, after going thru security, to the right as you are facing the park entrance.
I don't know how old your child is, but he will need to be with you if you request a GAC for him.
If he doesn't have his own ECV, there is information about renting ECVs and wheelchairs in post 2 of the disABILITIES FAQs thread. Be aware that the in park rental and most of the off site rental companies will not rent for use by a guest under 18. Guests younger than 18 can use their own ECV (no GAC needed), it's just that most rental companies do not want to be responsible for a minor using one.
Also be aware, all lines and attractions (except Swiss Family Robinson Treehouse and Tomorrowland Transit Authority) are wheelchair accessible to the point of boarding. Some attractions are not ECV accessible; those will have a wheelchair he can transfer to at the attraction if he doesn't think he can walk in line. You don't need a GAC to do that - just let the CM at the attraction know that he will need a wheelchair before parking the ECV.