I have kids attacking me and only read the first half but have to comment. DH and I are weight lifting junkies and I was competitively for a long time. We eat a LOT of protein and are always on the quest for lean muscle. I will try and get this out before kids get me.....
1) I totally agree on finding out if he is trying to get certain amounts of protein and carbs. For starters, he likely will find it cool that you care to help him meet his goals. Also, it can help you plan.
2) We combine our more expensive proteins with cheaper things. So take tuna...I mix a couple cans of tuna with old fashioned oats I grind up like bread crumbs, mix in egg and spices and make tuna patties. Ketchup and the kids even eat em. So the tuna is more costly, but eggs are cheaper and the oats are too and the expand to fill more space. And they are a great carb for muscle recovery. ;) Buy bulk vanilla whey protein. Best protein for muscle building...most bang for his buck. We cook oats (cheap) and mix in milk and vanilla whey (add banana, apple, nuts, whatever). We buy boneless skinless chicken on sale in bulk when it hits $1.49 or $1.99/lb. This is the meat we do not buy a lot of hormone free. Other meats we do. I also combine things like : 6 egg whits, 1c oats, 2 scoops of whey, 1-2 ripe bananas, 1/2c cottage cheese...blend and you can cook them into bodybuilder pancakes. :)
I think one convo to have, if the protein and such is important, is that to get those things down, he may have to sacrifice. Not everything will taste good. The price of being ripped! You will help him get what he needs, but if he has to gut down some peanut butter for his good fat, well, he can do it. He cant 6 meals and 200 g a day of marinated all natural chicken breast. ;)
In my years I spent personal training I put lots of cheap recipes plans together. If you want to message me after you talk to him, I can email you some of my recipes I have scattered on my computer that may help you and he get some ideas. I spent years putting them together and would love someone to get some use, lol. Protein shakes too....scoop of whey, milk, cottage cheese, honey, oats, blend, toss in something flavored/sweet.
PM me if you would like some ideas....I love helping teens that are working out. Just because it was so good for me at that age and well, I just think it is cool. :) And you are a Rock Star mom for caring enough to work on helping pull it together with him.