p.s. You might also ask your barn manager if she’d be willing to feed him a reduced calorie/starch/sugar grain (and then a small amount). Sweet feed is a no-no, but many pellets also have lots of extra calories that your horse doesn’t need. I use McCauley’s M30 and feed a small amount 1x day. Just enough that he gets the fortified nutrients that the other horses get from regular grain and regular grass allotments (which he can’t have), but not enough to add to his weight battle. This small amount also gives me the opportunity to mix in the supplements he needs (like magnesium–good for many reasons for overweight horses).
Other brands, such as Purina also offer low carb/low starch alternatives, but I find the McCauley’s M30 has the most protein per small serving (they also have an M10 for less protein). It is a similar cost to other quality grains, so it shouldn’t run your barn manager extra expense over her other boarders.