Hard time putting weight on my 27 year old gelding?

I have a 27 year old arabian gelding that is a rescue horse. He's 15.1 hands and last time I weight taped him he was 860 lbs.. He's SKINNY!!!! I currently have him on purina senior feed he gets it in the morning and evening, beat pulp morning and evening and he is at pasture right now for free grazing and I just don't think he's gaining as much weight as i would like to see. He is UTD on all his vaccinations and just had his teeth floated bout a month ago.. And his teeth were AWFUL when i got them done. Vet said worst teeth he had ever seen (they were really long)... Any tips on anything else i can do for him would be GREAT!! He's my love, such a people lover ..