Off the top of my head, my kit has:
My brain, with the training I have given it, and whatever is in my environment;
plus:
a couple wound care kits (
http://store.nols.edu/store/pc/WMI-Wound-Pack-p16.htm#.VHeOczCJOuY)
irrigation syringe (
http://store.nols.edu/store/pc/Irrigation-Syringe-p11.htm#.VHeOxjCJOuY)
EMT shears
latex gloves (several pairs if there's a chance he might be out overnight or longer in an emergency)
a small rite in the rain notepad and pencil or sharpie
triple antibiotic ointment
alcohol wipes
ibuprofen
aspirin
benedryl
a small rite in the rain notepad and pencil or sharpie
band aids
gauze pads
athletic tape
duct tape (for preventing blisters)
tweezers (I use the nice, real pointy ones that botanists and surgeons use, but the regular ones will work in a pinch (Ouch. bad pun, sorry.)
a needle and thread (needle for splinters, thread for repairing clothing)
a small rite in the rain notepad and pencil or sharpie
this book:
http://store.nols.edu/store/pc/Wilderness-Medicine-Field-Guide-3rd-Edition-p61.htm#.VHeU6zCJOuY , because if I am the one who is hurt and the person I am with does not have the training I have I want them to be able to figure it out.
I also carry the following things, but I separated them out as you may not want to put together my entire kit :)
Patient assessment bandana (
http://store.nols.edu/store/pc/Patient-Assessment-Bandana-p26.htm#.VHePvDCJOuY )
a flashlight
a watch (not a cell phone)
moleskin (for if I get blisters)
2nd skin (for burns)
safety pins
epipen (rx only. I'm not allergic to bees, but my partner is. I carried this before we learned he was allergic to bees though.)
pocket CPR mask
hydration salts
personal medications, etc.
A lot of people love the green soapy sponges, but I am always worried about them breaking open and getting soap everywhere, plus I usually restock my kit from random stuff that my MIL, who is a nurse, passes our way. You might want to consider them though.
All of the stuff on the website (WMI of NOLS) I'm sending you to is aimed at wilderness professionals. The training that WMI offers, though not as affordable as a first aid kit, is absolutely worth it. If your son does this work regularly, I would encourage him to get WFR certified. It looks great on a resume, and it has saved more than a few lives.
Be sure to sew the kit out of bright orange, red, or yellow synthetic fabric. I've found that using the awful cordura stuff that all the commercial kits are made of helps too. The more the kit looks like what people expect it to look like, the easier it will be to find in an emergency.
Oh, one last thing: My SO is an archeologist. the poison ivy may be a lost cause, depending on the job. My partner was on one job where the poison oak was so bad that they ended up making themselves walking sticks out of the stuff, as there was no point in trying to avoid it.