The line to wait on to get in is now the vax check line, but it moves fairly quickly. It spreads out the crowds hitting the metal detectors. I haven't seen lines of more than 2-3 people at the detectors these days. Where you're going to wait is at the gate. You now have to scan your tickets at a scanner instead of the ticket takers scanning them, so it seems like there's been a learning curve for people trying to get in how to use the scanners. I'll admit to getting scolded by a ticket taker for doing it wrong the first time.
Short of you getting someone on the phone to get an official answer from MSG, I'd tell you to bring his birth certificate. When you get to the head of the line, tell them your son is 4 (have him wearing a mask since he's going to have to wear one inside), show the birth certificate if they ask for proof, and ask for a manager if they continue to give you a hard time. It's been my observation that the people checking vax docs skew to the younger side (interns? temps?) so they may not give you too hard of a time, but doesn't hurt to be prepared.
