As Josh said, www.ddayadventurepark.com
will have a lot of information on there.
Some things to expect is this:
This is June, and it gets pretty hot over there generally. Make sure both of you drink plenty of water and stay hydrated. Especially your son, as the younger crowd tends to give it all they got and not know when they need to slow down and rest for a minute. Bring a camel back for both of you, because at some of the dead zones they have clean drinking water there that you can use to refill your camel back's with. I'd also suggest bringing something to eat that you can put in your pocket. Power bar's are good.
Secondly, in regards to cost, it depends on you really.
There are quite a few vendors set up selling markers, gear, and accesories so you might want some extra cash for that. Paint is if I remember correctly, $65 / case for the cheap stuff, and $75 / case for the premium. Then of course, you have the $70 registration fee.
Generally, when I go, I like to play a couple of the scenario games before the actual event, so I'll buy 3 cases of paint (I tend to shoot a lot).
So..generally speaking, for you and your son, if you are just playing the one day for the big game, and not buying anything from vendors, I'd say $300 or so for the two of you which would include the registration, 2 cases of paint, and a snack.
As far as a unit, that as well would depend on your play style, and how "in-shape" you are. You should join the 352'nd German infantry division though
We get to sit on the side of a hill and shoot at allies trying to climb up the hill for a couple hours
Special forces units tend to be more physical and have a lot more movement involved with them. You will be moving basically around the entire field going up and down hills. So if your not ready to walk in 90+ degree weather with a paintball mask on, loaded pods on your back, then your probably going to want to stick to a non special forces unit such as the 1'st ID, 4'th ID, 352'nd Infantry, 914'th / 916'th german infantry.
Hope that helps you out some.