If he literally cant eat anymore, or the food just isnt available and its cheaper (or significantly easier) to use a weight gainer, then either will do the job. Dont forget, weightloss comes from burning more energy than you're taking in. To stop weightloss is purely a matter of taking in enough calories to offset the extra work he's doing.
From the sounds of it, Muscle Milk probably would suit better with less crap and more protein.
Dont forget though, 500 cals from a shake is EXACTLY the same amount of calories as 500 cals worth of steak and veggies (as far as weight gain goes), and the real food comes with all sorts of extra good things in it.
Thats the point we're trying to make. If possible, use real whole foods to get the extra cals. If thats not a viable option, then flip a coin and go with muscle milk

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