First off, I'd like to say thank you for getting involved as a parent. We get a lot of bratty/whiny kids on here so this is definitely refreshing. Secondly, being involved as a parent is great and I fully support proper parental guidance/supervision when using airsoft (and paintball). Remember though these things do look like real guns so unless you're out in the sticks don't shoot it outside where public may see it (or where peeping toms could be walking by peering through your bushes/fence and then calling the police).
The only way you're going to get HPA is if you decide to go with "classic airsoft". And that stuff goes for $2000+ easily (at least inside Canada).
Also remember that most fields will have 18+ rules however some fields that have a "16+ with parental waiver and/or parent present at all times" aren't uncommon. There are some fields that allow younger but they are few and far between (the only ones I can name being "E.A.S.Y. Group", LowShotz, and Ambush Anonymous).
As for tinkering, that's great actually mechanical skills/aptitude is always a good thing to have.
ARES is actually a pretty good company so you should be fine with their products. Other companies I would suggest are Classic Army, KWA, G&P, RealSword, King Arms, VFC, and TM (Tokyo Marui) to a lesser extent.
Don't get me wrong TM is a great company with tight tolerances, great build quality and a high standard of quality control but they're of plastic construction (which could be a good or bad thing depending on how you look at it) and OOTB they come with a low FPS due to Japanese Law (which can easily be changed with a $10 spring upgrade).
I would seriously suggest that you work on getting Age Verified which just involves meeting up with an AV rep them checking you are who you say you are, and your DOB and then forwarding it to the admins/staff of the site to update your profile with an AV tag (done once a week or sometimes may take up to 2 weeks). That just opens up a wider market for you so that you might be able to find a great deal on a used packages or BNIB stuff. It's not 100% necessary but I would go through with it just to see what's on the other side (and trust me the other side is awesome).
As for budget, if you're both woodsball/milsim players then your BDU's, PB mask, barrel condom, and other stuff can transfer over so you can save some money there. I would say that you should budget $450-500 for a good used gun maybe up to $600-700 if you want something that's upgraded or has extras like extra mags, charger, optics, extra batteries, etc. So that brings it up to $1000-1400 to kit both you and your son with guns and then maybe another $200-300 to kit you both up with vests/chestrigs and other random accessories if you so desire.
PS: Airsoft "snipers" don't really have that much of an advantage over other players. The range of the guns are fairly similar unless you heavily upgrade the sniper rifle and even then you only get one shot while the other guy can unload a hailstorm of BB's in your general direction. Remember you only need one hit to get someone out, it's not like pistols do less damage than assault rifles, or sniper rifles, a hit, is a hit, is a hit. Technically automatic rifles can be more "accurate" over the same period of time as a sniper rifle because they can just fling more BB's downrange in the hopes of hitting someone. Also automatic rifles AEG's) can be upgraded just like sniper rifles and be able to float out BB's downrange across the field like a sniper rifle can.
Good luck, I hope this info helps you out.
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