Unfortunately “I have nothing to offer, so will come work for you” is about as likely a pass to employment and training courses within the armed forces as it is with any employer. (And as his shiny new way of shutting down criticism and nagging from concerned relatives, it’s probably even less effective).
When I signed up (at 15, so I’m a bit out of date now) it was drummed into us that there were 14 applicants for every place within our cohort, and we were expected to display the necessary skills and enthusiasm - or stop wasting everyone’s time. It’s very much, we’re paying you a wage, providing a shedload of (often insanely) expensive equipment for you to use, now show us why!
If he’s genuinely serious about this, it could be the making of him though. I signed up with a trade in mind, and the technical corps - so Engineers, logistics, REME (electrical and mechanical engineers) are where he is most likely to A learn a trade, and B stick with the army as a career. Infantry regiments can be less academically choosy, but obviously more physically demanding (from day 1) and less likely to be his ‘until retirement’ plan. Don’t ignore the cavalry - riding and mechanics, or marines (small boats and diving). The technical corps are looking for intelligent and motivated soldiers, though, so grades may count more than you’d both hope. The more thought he puts into choosing where to apply the better - recruiting offices are looking for people who’ve thought this through, and see a career not an expensive gap 4year experience! Get him in for an initial chat right away, it could either nip this in the bud, or set him up well - AND I’d expect them to tell him “revise, boyo!”.
Bullying… SO cohort dependant. My fabulous experience leans me towards, if he’s affable and keen, chances are he’ll be ok. A gobby or lazy lad with low self motivation or confidence would struggle. Army life is communal, and drill sergeants are trying to get themselves the best quality colleagues for a frequently difficult and dangerous job. Teamwork is everything. You get put into teams and expected to pull together. Letting down your team is generally THE sin, although if you work hard and still don’t achieve, meh, there’s extra pt and training for that.
Basic training is for all soldiers, so they can, well, soldier! Get fit, look after themselves (clean, cook, camp) map read, shoot, first aid, swim etc. Fitness to start with makes it SO much easier. His fitness will be expected to improve, but everything is easier if you’re not constantly exhausted and struggling to keep up, so why make life difficult? They’re pushed to see who really wants to be there and who thought it was the easy option.
Passing out gets you through to your Corps/regiment training. Skills, trades, more drill and polishing for the cavalry and infantry, specialist weapons and equipment training. The kit can be impressive and bloody good fun and they’re let loose on it at a point where civilians are often still at school. THEN you’re to on your job.