Hmmm. Dirk subtly force feeding Dave.
Dave is visibly underweight, not by any diet or because he particularly wants to be, but because he's hyperactive and fidgety (always burning a shit ton of calories), and because food hoarding is a difficult compulsion to kick after a lifetime of food insecurity. Dirk wouldn't mind that, but Dave is always complaining about how he's cold, and Dirk sometimes has to slow down for Dave when Dave gets dizzy, and sometimes when Dave is shirtless his skin looks translucent and grey from how undernourished he is. It makes Dirk feel awful, especially since he can see the easy solution here. He's not sure if Dave would accept his help if asked, so Dirk decides to go with the philosophy of it being better to ask for forgiveness than permission.
Dirk would always invite Dave over for protein shakes after Dirk works out anyway, so he figures that would be an easy place to start. Dirk spends so much time and effort and money finding a bulking powder that tastes good and works well (he has a bulking powder of his own, but it's just the cheapest one he could get his hands on and he wouldn't make Dave drink something that tastes like artificially sweetened cardboard). He spends even more time and effort every time he makes the shakes, since now he makes them separately (which he claims is just because his blender's motor has started acting up and can't take the strain of making two huge, thick smoothies at once). He makes his own how he usually would, then he makes Dave's off of the same general formula that Dirk uses when he is bulking. Frozen banana instead of ice+banana, bulking powder instead of protein powder, whole milk instead of skim, and an extra spoon of peanut butter. They look pretty similar in texture and color, and any time Dave thinks to comment on how Dirk's has visible ice chunks and looks runnier, Dirk just shrugs and says he wanted to give Dave the one that came out better. Dirk starts working out 6 days a week instead of 5 so that he can invite Dave over one more day each week and make sure that Dave eats that much more. Dave never turns it down, free food is free food, and Dirk's shakes do taste better than any sort of prepackaged protein shake Dave has tried.
It works. Dave does gain a bit of weight. He finally gets into the healthy weight range. Dirk learns about that because one time that Dirk was hanging out Dave, Dave gave him the news that he weighed himself and he was at a healthy weight. Dave is genuinely ecstatic about that, since he hadn't been at a "healthy" weight since infancy. Dirk sees no issue in letting Dave think that that was his own doing, after all boosting Dave's confidence in his ability to get to and maintain a healthy weight is important for this to work long-term. Dave plateaus though; he hits a point where just the shake substitution isn't enough for him to keep gaining weight. And technically, Dirk could stop with that. Dave is in the healthy range, albeit barely. His cheeks don't look so sunken in, he seems a lot happier and his energy seems genuine and positive, not just nervous energy and tension. But Dave still sometimes complains about being cold in circumstances where he really shouldn't be cold, and Dave's skin still looks grey occasionally even though he's looking a lot better overall. Dirk thinks that Dave would be better off gaining a few more pounds.
The next time that Dave's stomach growls while Dirk and him are out and about, Dirk doesn't give him the granola bar he usually gives Dave when Dave gets hungry and hadn't brought food. Dirk has a meal replacement bar that he slides into Dave's hand instead. That's 400 calories instead of just 100. Dirk is prepared to say that that's just what he had on hand, but Dave doesn't even seem to realize it's a meal replacement bar instead of a protein bar before he tears open the wrapper and shoves half of it in his face. Dirk relaxes just a little. He makes sure he has those on him all the time, and when he's given Dave basically every flavor that brand has, he makes sure he always carries Dave's favorite flavor.
Dave gains a little more, until he's in the upper end of a healthy weight. He looks better. He looks happier. He doesn't complain about being cold when he wears a t-shirt in 70 degree weather. He goes out with friends more often instead of just texting them from his apartment. He playfully tussles with Dirk a lot more. Dirk doesn't feel scared of breaking Dave's bones when they rough house anymore. He didn't even realize that fear was due to Dave's weight until the fear was gone. Dirk still wants to fix Dave's food hoarding compulsion, but for right now, he feels happy too.