I had no luck with purina senior, I switched my 36 year old arab/quarter to triple crown senior and have seen a significant improvement I have also added alfalfa pellets and hard keeper solution to his diet. He is by no means fat but he looks a lot better then he did this time last year. Also feeding the food soaked will help the oldies with poor/no teeth digest it better
Here is the thread with pics of my oldie and his weight gain since switching from purina senior How much more weight does he need to gain about?
I forgot to add that if his teeth are poor he may not be able to eat the hay so getting chopped hay or soaked hay cubes may also help put on the lbs...do you have a pic to we can see how much weight he needs to gain?