First of all, I think its absolutely great what you are doing. I can relate on a very personal level to your situation, because a year ago I was 5'7" at 290 pounds, and I now stand at 5'7" 175lbs.
I didnt start with someone helping me out, I just kind of did it on my own. Looking back, I think I have something to offer given my unique perspective.
I think your diet is great. Perhaps instead of giving him a specific diet, give him some guidelines.
1) Choose a decent portion of lean meat, just like you said.
2) Eat freely of vegetables.
3) Leave the starchy carbs for morning, and after excercise.
When someone is really big, they are used to eating LOTS of food. When I started, I would eat a normal portion of protein, and eat a TON of veggies. This way, I was still eating till I was full, and not doing much damage calorically.
For a while, he may need this. People "our" size (formerly), dont differentiate between satieted and "full". So, initially he will need to get the same feeling of being full, otherwise here comes the snack machine on the way home from MicroBiology.
You may want to give him a list of "acceptable" foods. This way, when you arent there and he goes to the line, he will have a mental list of whats ok to eat if something you said to eat isnt at the bar.
I started on a LOW carb style diet. Looking back, Im glad I did, because even though it wasnt the most healthy thing (lots of sat fat in my case) it helped me lose weight, which led to it being easier for me to cut out bad foods because of the reward.
As far as training, I started with full body workouts of 3x10, and lots of isolation movements. I lifted in highschool, so I was more comfortable around the gym. I took 2 years off after graduating from working out, and gained 60 pounds. When I started back, I used machines for a long time. I had to get a feel for what I could do, and when my body was about to reach failure.
I cant say much about the excercise program, but as Im sure you know, almost anything he does is going to make a vast difference. Start with ideal and work back to whats possible. I wasnt comfortable squatting out of a smith machine for a long time till I got my weight down.
As far as cardio, thats a conundrum for a hefty fella. I could run much faster than my body could handle, meaning my legs where still going,but my heart was about to beat out of my chest. Looking back, I wish I had done more biking and swimming instead of running. Easier on the joints.
Regardless, its so commendable that you are doing this. When I started, the worst thing was going to the gym and being the fat kid. Your support and encouragement with him will make all the difference in his progress. As long as he feels he has you in his corner, he will be so motivated. Good luck, and let us know how it goes!