I personally don't see an underweight horse. I see hipbones, but given that this horse is Arab/Saddlebred, hipbones like that aren't completely unusual.
I don't see rib, at all, and his tailhead is not protruding - both things you would see if his protruding hips were an accurate indicator of his weight.
He does appear to have a bit of atrophy in his topline/wither-area but, given how high his withers appear to be, I'm not totally stunned about that either. He does have a thin neck, but, again, given his breeding, that's something I'd expect.
Personally, I wouldn't want this horse to lose a lot more [he looks to me like he could even lose a bit more without becoming ribby..not that he should lose more] but I do not think he's underweight here.
My gelding is built somewhat similarly. I don't know his breed [best guess is some kind of Arab cross, but who knows] and he is hip-y as well.
He's standing a bit weird and he's a muddy mess in this picture, but you can see his hips -
Here he is more recently, and cleaned up:
You can barely, barely, see a hint of rib which is what I want to see, given that this horse works everyday and tends to be a very easy keeper. If he were a harder keeper, I might not want to see rib at all [personally].
For reference, my gelding used to look like this, because I mistook his hips as an indicator of weight:
Anyway, I think the OP's horse is deceptive in the weight department and I have been there. I think he looks true to his breed, which is a good thing!
I think the OP should keep doing what she is doing, up his feed if he continues to lose, but don't worry about it too much. He looks good!