I'm not going to recommend any weedkiller at all - Roundup or glyphosphate, which is most people's weedkiller of choice, is proving to be much more damaging in the environment than any of us was ever lead to believe, and despite the manufacturer's promise that its inert in the soil and just disappears, this appears to be untrue.
I recommend a decent fork and a bit of elbow grease - weedkillers don't even work that well anyway, particularly not on woody based plants like brambles. For those, if he can't dig out all the roots, he may need to poison the woody stumps with something like SBK, poured into holes he's drilled into them. If its a large area and he hasn't the time to tackle it all, thick black plastic sheeting from the garden centre will work a treat - needs to be firmly pegged down over the area and left for a year - by which time everything underneath will be dead.

I like your thinking Bamboo - I hate using weed killers as well and reclaimed an old garden just by digging out the brambles and weeds.

I'm in the process of doing the same - just cleared several square feet with a lot of sweat using a saw, a fork, long handled cutters and a crowbar and wearing gloves. There will be some regrowth from the roots I couldn't get out but producing new shoots seems to shorten the roots in the ground and they come out easier when attacked a second time a few weeks later.

If your son uses weedkiller on the jungle he'll just have all the same plants to get out and burn or shred and compost. Better to attack them with sharp implements at least couple of times first and save any weedkiller for persistent regrowth.
Nettles are a sign of fertile soil - and if you can't grow weeds, you won't grow anything else!
Lots of the top growth of weeds can be composted, but not bindweed & don't compost the roots of docks, brambles, nettles and other "good rooters".

I don't recall using weed killers to clear a large plot of land of every type of rubbish that could be thrown by surrounding lazy householders and weeds also. It was a total jungle. Jusy heavy duty loppers( still in use after about 30 years) a sharp axe, old fashioned saw, elbow grease and some very large bonfires (a long way from surrounding houses) . Lots of trips to tips and stashing in skips. Lots and lots of 'youthful' energy ,enthusiasm and patience. A year or so later and one large hover mower and we had the beginnings of our orchard garden. No parental help and some help from friends. Oh I forgot a large bulldozer to dig a large hole and bury some of the 8 mattresses that had been thrown there!!!!!!! People can be so messy.

Thanks everyone, I will pass on your 'anti-weedkiller' comments which I do agree with. I asked the question as we have only used weedkiller on our laurel stumps-avoiding it in all normal weed situations, but wondered if there was a 'quick-fix' solution which I guessed would prob be a chemical one. But may be not!