Hi. I live in NYC and use Penn Station pretty frequently, although--disclaimer--I have never used a porter to check my bags. But I've never seen a porter at the entrance on 7th Avenue. Apparently there are often porters at the 8th Avenue entrance. Take a look at this extremely useful information:
http://jasongibbs.com/pennstation/ which I found with only a little searching.
You don't say how you're getting to Penn Station. Taxi? Subway? Car? If it's the subway or LIRR, then you don't have much choice where you're going to get off your transport into the station, but if you're taking a car or taxi, then make sure to be dropped off on 8th Avenue. Apparently the former service drive is now closed to vehicles. Not sure why.
Are you traveling coach? Or do you have a roomette or room? If you're in coach and you could possibly afford it, I highly recommend taking a roomette. This is a very long trip--about 24 hours and sometimes longer if there're delays--and it's far more comfortable to have your own space. It doesn't really make up for the price difference, but you do get all your meals in the dining car included with the cost of the roomette and also you can use the Acela Lounge in Penn Station while you're waiting for your train. The lounge is quite nice and they really take care of the people who're traveling long distances. At least, that's been my experience.
Of course only you can judge what your son can handle and also what you can afford. Just be aware that it may be difficult to sleep in coach during the night, since it's not exactly silent.
DH and I always use the Orlando station and it's been absolutely fine. I know that a lot of regular Amtrak riders to WDW seem to prefer the Kissimmee station and it may be that it's better if you're in coach. Really, I don't think it makes a huge difference.
Hope this answered some of your questions.
Have a magical trip!