There is a lot that goes into the equation of developing and maintaining a good topline, especially in a senior horse, and one that appears to have a longish back.
The first thing to consider is whether or not he’s getting the recommended amount of his feed. You say a scoop AM and PM, but how much does that scoop of feed weigh? You might be falling short of the recommended amount, which should be listed on the bag.
The second thing to consider is that he might need a feed tailored to his age. A senior feed might help him maintain his body condition. The first place you’re going to notice a horse’s loss of condition is over their topline. So his lack of coverage there is an indicator that he’s not getting enough calories. Triple Crown Senior, ProElite Senior, Nutrena ProForce Senior (I think that’s what it’s called) can provide more calories (still need to be fed at recommended levels).
It sounds like you’ve got good hay if it’s an alfalfa mix. Alfalfa is great for adding protein and protein builds muscle, which helps the topline. The other way of adding protein could be a ration balancer like Nutrena’s Empower Topline Balance. For horses that are easy-keepers, this can be their only “feed” along with hay and pasture. For a senior horse like yours that needs more calories, it can be top-dressed on his feed to add protein.
There are also amino acid supplements like TriAmino that can be supplemented if you don’t want to go the ration balancer route. These provide essential amino acids that help with muscle development.
Finally, how he is ridden also has a lot to do with the musculature over his topline. If he’s ridden with his head high, back hollow…that can create the kind of musculature you’re seeing too. He needs to be encouraged to stretch over his back and lower his head and neck, stepping under from behind to propel himself along. Basic dressage lessons could help your daughter learn how to ride him in such a way that it actually improves his conformation.
Which actually brings up another point. As horses age, they get aches and pains just like we do. He may not look “lame” to the naked eye, but if he’s got any creakiness in his joints (hocks are common) it can definitely cause him to lose condition just because he’s in a chronic low-level of pain and is compensating for that. So, that’s something to be aware of too.