Some additions to the above.
I'd add switch the magnesium with ZMA, and add some melatonin as well. This will increase testosterone, so it will help improve the hormone balance in a positive way.
The fact that he walks around all day long is great, especially if he lifts heavy things from time to time. The bad part of working in an auto shop is going to be the exposure to all the toxins, from not just car exhaust, but all the other stuff like motor oil, etc. Plastics and things like BPAs are estrogenic and can cause weight issues as a side effects. So he'd need to take a look at what he's exposed to there and see if he can limit exposure. DIM might help counter some of this.
The diet soda thing is horribly bad. Aspertame is an excitotoxin which does a lot of damage to all sorts of tissues including both brain and organs, and is a carcinogen. The stuff breaks down to formaldehyde and can and do cross the blood-brain barrier.
Regular soda with sugar would be better health wise (though the sugar will prevent weight loss.) Soda that's made with high fructose corn syrup will make it much worse as it will cause non-alcoholic fatty liver, which does the same damage as alcohol.. So in my book, I'd rather drink alcohol (but again, this will prevent weight loss.)
Speaking of which, I'd remove all fruit and especially juice (except maybe lemon/lime juice in NorCal margaritas.)
The caffeine in the soda will also keep him from sleeping, and thus prevent weight loss.
This is very critical if the fat pattern is love handles since those are cortisol related.
In my own case, I noticed a huge improvement within one month in terms of weight loss - I lost 5lbs and gained muscle. But I do lift weights almost every day (different muscle groups every other day, and sometimes if I feel a bit burned out, I skip a day entirely.)
If you and he are actually worried about weight loss, get a body fat monitor, one that will also tell you muscle mass and water weight. Better ones come in the form of a digital scale. He'd step on it with bare feet and it will tell him body fat percentage as well as muscle mass.
Weight in LBS by itself is a useless measure because you don't know if you're holding on to water, or gaining muscle. Mine tells me fat as a percentage of body weight, so I do a bit of math to figure out the actual pounds of fat, pounds of water (I know, water is supposed to be measured in gallons or liters), and pounds of muscles and track these daily and track these in a spreadsheet (so it does the math for me). This way I know exactly what's going up and what's going down and can take notes such as "Well, yesterday I did a leangains style IF, or yesterday I had a cheat meal because I went out and ate non-paleo and had beer, or yesterday I had a lot of energy and worked out a lot more and I see a slight increase in muscle mass, etc."
To me, seeing this progress is very encouraging. You don't have to do this, it's just a motivator.
Once he gets all the variables down (especially stress), he can accelerate the weight loss with intermittent fasting 1-2x a week. It sounds painful, but it's very easy if you do it leangains style. He'd have dinner before 8pm, then skip breakfast and have lunch the next day. That's a 16 hour fast and he's all set.