I love beet pulp for helping add weight.
I would suggest adding a senior feed as well....its higher in protein then other pelleted feeds, and he needs muscle just as much as fat at this time.
Also if possible increase his hay intake over night, you say he gets only 2 flakes at night, but Id up that to what he gets in the am....just cuz he is on grass all day doesnt mean he doesnt need to eat hay at night :)
My horses have hay in front of them at all times.
Horses are like chips...you can never just have one