A lot of fields are strictly 18+, and most that accept minors are 16+, so the biggest challenge I see is finding a field that will allow you to play with your son.
But best tips are, don't cheap out on eye protection, use military grade if possible. Very good boots, you don't want to sprain an ankle. If you play with a good group of experienced players, having radios to call out enemy positions and movements is a must. Once you have all that you are ready to play. Having a gun is optional from this point, but useful.
Don't listen to all the crap you hear on facebook. You don't need to modify everything in your guns, a lot of cheap upgrade parts are actually downgrades. That cheap chinese "upgrade" barrel will likely have worse accuracy than your stock barrel, whatever the guys on facebook or reddit say. Quantifying improvements in gun performance is hard, and the placebo effect is strong to justify their purchases.
Stay away from HPA, you will look like a paintballer, and the performances are not any better than with a well tuned AEG or bolt action. It's just a giant waste of money, but it's being peddled a lot right now.
The trick to not be seen is to not move, your camo is useless as soon as you move, and sometimes just turning your head to look in another direction is enough for an enemy player to spot you when you are lying down behind a bush. If you can see them, they can see you.
For other tips, well, just ask, we'll be happy to provide!