grand666 - I've been a super obese trainee my entire career, both shorter and MUCH heavier than your new guy. I'm currently squatting and deadlifting around 565, broke 300 on the bench, and press around 250 - not a brag, but illustration that the question is probably less about how overweight he is, and more about his anthropometry
I had problems early on that sound similar to what you're describing - you'd do well to assess his arm proportions. Specifically his bicep vs. forearm lengths. I had issues with squat grip for a long time and almost dug myself into a nasty rotator cuff injury by being dogmatic about getting a textbook grip on the bar. I was doing band stretches and PVC stretches and it made the problem worse until it resulted in a layoff.
If he has short biceps and very long forearms, he will need a wider-than-textbook grip. It still needs to be dialed in enough to create the arm tension and shove the bar into the back above the scapulae, but it doesn't need to be inside the rack cups if he can't make it. A lot of people tried to force me into this position to no avail, and I used an extra-long squat bar for a period of time when squatting in racks of unfavorable proportions or design to allow me to grip the bar outside the rack cups.
I find stretches against the rack, with my hands somewhat at squat height but a little lower with my elbows pressed against the rack, does a lot better job than the broomstick/PVC method did for my particular anthropometry. If he has the same problem I did, he will also need to rack the overhead press on his arms, possibly above his chin - my forearms are so long I biologically cannot rack cleans or presses on my shoulders/across sternum.
The backfat issue is just something he needs to learn to deal with. It makes finding the proper position over the scapulae difficult, but over time I've found the initial issues went away as my body adapted. I find in general, as an overweight trainee, that light weights are harder for me to manage with good form than heavier weights - this sounds counter-intuitive and isn't necessarily something I'd advise anyone to do, but more weight on the bar forces me to use better form whereas a lighter bar I can manipulate with less effort so I can get lazy under it. This obviously has limited returns with bad form, you want it heavy enough that he can't cheat, but not so heavy if he's rounding his back even then that it won't injure him. 65lbs for a 240lb guy sounds exceptionally light at first glance.
Regarding the bar sliding up, it really sounds like a grip/anthropometry issue - very similar to the problems I had. I'd assess those first. Regarding the rounded back, a queue that worked for me was "lead with my head or I'll be dead" and imagining pulling myself head-first straight up out of the squat, doing so helped me keep my chest up and my head/shoulders tight to prevent the bar from rolling up on me.
Hopefully something in my reply helps.