15.8 is a med protein. 17 + is higher protein. there will be some proteins in the pasture grass and the first sign of excessive protein would be very dark urine.
what are the other percentages in the bagged feed ? fat / roughages etc...
The oil is a good thing, it takes a while to put on weight .
As for the insects, can you put something on him to discourage them ? swat or even vaseline, fly spray .. if they keep biting him, he will keep getting the edema.
he sounds like he needs the extra nutrition, so I would not change feed on him.
feed made for senior horses is good for any horse that needs to add weight. It is easier to digest .